The differential protection for power transformers features less ratio of proper actuation and slow response， because it severely suffers from false differential current caused by core saturation. Investigation shows that transformer core saturation needs certain time， so emergence time of abrupt differential current of inrush is lager than the time of abrupt change phase voltage. And it can be utilized to distinguish internal faults from external faults. A new criterion for transformer differential protections based on abovementioned time difference is proposed. Furthermore， a novel filter algorithm is designed in order to ensure fast and accurate detecting， effective noise-repressing as well as high-speed response during the instant of variation emergence. The EMTDC-based simulation， which covers various fault situations， is analyzed， and the test results confirm that the criterion can effectively prevent the differential protection from mal-operating and guarantee the high-speed response to the internal fault as well.