For the substations of industrial and mining enterprises, when the equipment is in installation or debugging stage, it is often unable to organize load for the main transformer. So, traditional method can not be used to check the correctness of transformer differential protection phase. Taking a 35 kV substation in Xuchang city as an example, with the theoretical calculation and analysis, this paper proposes a method of measuring main transformer differential protection phase and the secondary circuit phase of transformer protection CT. As a result, this method solves the problem of phase measurement of protection CT secondary circuit of transformer differential protection. For the substation where two transformers are in operation, this method can also be used to measure the current phase of the differential protection of the transformer.