Several cases of unusual mal-operation of the differential protection for transformer occurred in the power gird in 2016, threatening the stable operation of the power grid and the reliability of the power supply seriously. The mal-operation reason is researched in detail based on the mathematical analysis, the digital simulation and the field recorded fault data analysis. It is disclosed that the P-class CT transient saturation leads to the mal-operation. On this basis, this paper analyses the performance of the CT saturation method, and discusses the CT selection principles. This paper aims at attracting the attention on the problem of the CT saturation and proposing the reference of the analysis method and the countermeasure for this problem.