The polarity checking of zero-sequence differential protection (Y side CT of transformer) is difficult because there is no zero-sequence current in the normal operation of the power system. Research shows that the traditional method requires load current, which can not be applied to the scene of no load. To solve this problem, a new method is proposed. The method uses the zero-mode component of inrush current generated by no-load closing, and calculates the waveform similarity of self-produced zero-sequence current and neutral point current at the Y side, adopts the “hypothesis-test” method, traverses all possible polarity error cases, and realizes the CT polarity calibration of zero-sequence differential protection. To improve the adaptability of this method in extreme inrush conditions, the Euclidean distance is introduced. The proposed method is simulated with PSCAD/EMTDC4.6 and Matlab, and verified by the on-site recording data of a 500 kV transformer of the China Southern Power Grid. The results show that the proposed method can identify CT polarity error type of zero-sequence differential protection without load current, and can effectively prevent mis-operation. This work is supported by National Key Research and Development Program of China (No. 2016YFB0900600) and Science and Technology Project of China Southern Power Grid Co., Ltd. (No. 000000KK52180016).