The utility model discloses an it optimizes structure effectively to reduce transformer stray loss's case lid, including that case lid body, triangle -shaped separate magnetic steel plate, three -phase high tension lead and three -phase low -voltage lead, having seted up three high tension lead mounting hole and three first mounting hole on the case lid body, three first mounting hole is triangle-shaped and arranges, triangle -shaped separates magnetic steel plate to be fixed the lower surface of case lid body, triangle -shaped separate magnetic steel plate's three angle one -to -one ground and have seted up three second mounting hole, and three second mounting hole and the setting of three first mounting hole one -to -one ground, every second mounting hole and corresponding low -voltagelead mounting hole of first mounting hole constitution, three -phase low -voltage lead one -to -one ground alternates in three low -voltage lead mounting hole. The utility model discloses an it optimizes structure effectively to reduce transformer stray loss's case lid shortens three -phase low -voltage lead width between centers, can reduce fuel tank cap vortex magnetic resistance to reduce fueltank cap eddy current loss.