In　order　to　solve　the　problem　of　transient　stability　assessment　deviation　caused　by　the　imbalance　of　power　system　sample　quantity　and　quanlity,　starting　from　the　training　process　of　assessment　model,　the　gradient　norm　of　samples　to　model　parameters　is　obtained　by　the　pre-training　model,　and　the　mean　ratio　of　gradient　norm　is　introduced　to　quantify　the　imbalance　of　samples.　Compared　with　the　prior　information,　the　mean　ratio　of　gradient　norm　comprehensively　considers　the　imbalance　between　the　sample　quantity　and　quality.　An　imbalanced　correction　method　based　on　the　cost-sensitive　method　is　proposed,　which　is　used　to　improve　the　assessment　preference　of　the　model　and　realize　a　preferable　correction　effect.　The　simulative　results　of　IEEE　39-bus　system　and　East　China　Power　System　verify　the　effectiveness　of　the　proposed　method.　©　2024　Electric　Power　Automation　Equipment　Press.　All　rights　reserved.