As an important protection component of power transformer, the correct operation rate of pressure relief protection has been at a low level, and it is easy to misoperate under external short-circuit. Pressure relief valve is a mechanical action device widely used in China, which is not well adapted to complex working conditions. In order to improve the efficiency of pressure relief valve, this paper starts with the misoperation mechanism of pressure relief protection caused by external short-circuit, establishes the mathematical model of pressure transfer in transformer tank under external short-circuit impact, extracts and analyses the corresponding characteristics of pressure change in tank, so as to provide a basis for the improvement and threshold setting of pressure relief protection.