Abstract: In view of problems that traditional main protection of transformer cannot identify inrush current and internal fault current of transformer, the paper proposed a new criterion of main protection of transformer based on dead angle of differential current. The criterion uses sym4 wavelet transform to process inrush current and internal fault current of transformer and obtains obvious difference characteristics of dead angles of the both under d1 scale, so as to identify inrush current and internal fault current of transformer correctly. The simulation result validated effectiveness of the criterion.