Based on the differential pressure method of gas flow measurement,a new measurement method of samll gas leakage flow,the constant pressure constant volume differential pressure method was proposed.In order to eliminate system errors,the method controls the air inlet pressure to be constant before and after measurement,and can achieve the highprecision measurement of small gas leakage flow of aeroengine sealing parts.The measurement method of small gas leakage flow was introduced,including the measurement principle,the selection of high precision temperature sensor,pressure sensor and data acquisition card.That the measurement method has high accuracy was proved by seal leakage test.The feasibility of measureing small gas leakage flow by constant pressure and constant volume differential pressure method was verified by experiments.