Based on the sudden decline of the secondary current after current transformer saturation， a new saturation criterion called current-dropping method is presented. It can be concluded from the simulation results on EMTP that the differential protection can be effectively blocked during the external fault， while they are quickly opened during the internal fault due to the current-dropping method. Meanwhile the problem of opening the differential protection during the transitional fault of the same phase with current transformer saturation is also solved. The proposed method is a promising current transformer saturation criterion with predominant performance， clear characteristics and simple logic.