Traditional converter transformer differential protection is easily affected by DC bias and fault transition resistance. These may cause the rejection of differential protection. Thus a criterion of converter transformer differential protection based on the time-frequency matrix elements and phase angle discrimination of a Current Fault Component (CFC) is proposed. In the case of an internal fault of the transformer, the CFC direction detected by Current Transformers (CT) on both sides of transformer is the same, while in case of an external fault, the CFC direction detected on both sides is opposite. The CFC time-frequency matrix is obtained by an S-transform. From the phase angle of the phasor obtained by summing all elements in the time-frequency matrix at the fault time, the direction of the CFC is distinguished, so as to realize the fault identification in the converter transformer region. The simulation results show that the proposed protection criterion can effectively identify the internal and external faults of the converter transformer under various fault conditions, and still has high reliability in the case of DC bias and fault transition resistance. At the same time, it can effectively reduce the misoperation risk of differential protection. This work is supported by the National Natural Science Foundation of China (No. 51777119).