diff --git a/app/models/merge_request.rb b/app/models/merge_request.rb
index dd9f1a7c85b..ea3cf1cdaac 100644
--- a/app/models/merge_request.rb
+++ b/app/models/merge_request.rb
@@ -101,7 +101,9 @@ class MergeRequest < ActiveRecord::Base
   validate :validate_branches, unless: [:allow_broken, :importing?, :closed_without_fork?]
   validate :validate_fork, unless: :closed_without_fork?
 
-  scope :by_branch, ->(branch_name) { where("(source_branch LIKE :branch) OR (target_branch LIKE :branch)", branch: branch_name) }
+  scope :by_source_or_target_branch, ->(branch_name) do
+    where("source_branch = :branch OR target_branch = :branch", branch: branch_name)
+  end
   scope :cared, ->(user) { where('assignee_id = :user OR author_id = :user', user: user.id) }
   scope :by_milestone, ->(milestone) { where(milestone_id: milestone) }
   scope :of_projects, ->(ids) { where(target_project_id: ids) }
diff --git a/app/services/merge_requests/refresh_service.rb b/app/services/merge_requests/refresh_service.rb
index e4056306bc4..0a9563ed7e7 100644
--- a/app/services/merge_requests/refresh_service.rb
+++ b/app/services/merge_requests/refresh_service.rb
@@ -55,8 +55,9 @@ module MergeRequests
     # Refresh merge request diff if we push to source or target branch of merge request
     # Note: we should update merge requests from forks too
     def reload_merge_requests
-      merge_requests = @project.merge_requests.opened.by_branch(@branch_name).to_a
-      merge_requests += fork_merge_requests.by_branch(@branch_name).to_a
+      merge_requests = @project.merge_requests.opened.
+        by_source_or_target_branch(@branch_name).to_a
+      merge_requests += fork_merge_requests
       merge_requests = filter_merge_requests(merge_requests)
 
       merge_requests.each do |merge_request|
@@ -157,13 +158,14 @@ module MergeRequests
     def merge_requests_for_source_branch
       @source_merge_requests ||= begin
         merge_requests = @project.origin_merge_requests.opened.where(source_branch: @branch_name).to_a
-        merge_requests += fork_merge_requests.where(source_branch: @branch_name).to_a
+        merge_requests += fork_merge_requests
         filter_merge_requests(merge_requests)
       end
     end
 
     def fork_merge_requests
-      @fork_merge_requests ||= @project.fork_merge_requests.opened
+      @fork_merge_requests ||= @project.fork_merge_requests.opened.
+        where(source_branch: @branch_name).to_a
     end
 
     def branch_added?
diff --git a/changelogs/unreleased/do-not-refresh-main-when-fork-target-branch-updated.yml b/changelogs/unreleased/do-not-refresh-main-when-fork-target-branch-updated.yml
new file mode 100644
index 00000000000..12b1460f388
--- /dev/null
+++ b/changelogs/unreleased/do-not-refresh-main-when-fork-target-branch-updated.yml
@@ -0,0 +1,4 @@
+---
+title: Do not reload diff for merge request made from fork when target branch in fork is updated
+merge_request: 7973
+author:
diff --git a/spec/services/merge_requests/refresh_service_spec.rb b/spec/services/merge_requests/refresh_service_spec.rb
index bc340ff9d3c..7e3705983fb 100644
--- a/spec/services/merge_requests/refresh_service_spec.rb
+++ b/spec/services/merge_requests/refresh_service_spec.rb
@@ -126,17 +126,27 @@ describe MergeRequests::RefreshService, services: true do
     end
 
     context 'push to fork repo target branch' do
-      before do
-        service.new(@fork_project, @user).execute(@oldrev, @newrev, 'refs/heads/feature')
-        reload_mrs
+      describe 'changes to merge requests' do
+        before do
+          service.new(@fork_project, @user).execute(@oldrev, @newrev, 'refs/heads/feature')
+          reload_mrs
+        end
+
+        it { expect(@merge_request.notes).to be_empty }
+        it { expect(@merge_request).to be_open }
+        it { expect(@fork_merge_request.notes).to be_empty }
+        it { expect(@fork_merge_request).to be_open }
+        it { expect(@build_failed_todo).to be_pending }
+        it { expect(@fork_build_failed_todo).to be_pending }
       end
 
-      it { expect(@merge_request.notes).to be_empty }
-      it { expect(@merge_request).to be_open }
-      it { expect(@fork_merge_request.notes).to be_empty }
-      it { expect(@fork_merge_request).to be_open }
-      it { expect(@build_failed_todo).to be_pending }
-      it { expect(@fork_build_failed_todo).to be_pending }
+      describe 'merge request diff' do
+        it 'does not reload the diff of the merge request made from fork' do
+          expect do
+            service.new(@fork_project, @user).execute(@oldrev, @newrev, 'refs/heads/feature')
+          end.not_to change { @fork_merge_request.reload.merge_request_diff }
+        end
+      end
     end
 
     context 'push to origin repo target branch after fork project was removed' do