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At present, the test index of air leakage is not uniform in different standards in China. Constant pressure difference method and tracer gas concentration decay method were used to test air tightness characteristic of refrigerated transport unit. The variations of air leakage were tested under conditions of different pressures (50Pa, 100Pa, 150Pa, 200Pa and 250Pa), different velocities (20km/h,40km/h,60km/h, 80km/h and 100km/h) and different agings (0, 71000km, 126000km and 175000km). The matching relationship between different parameters was analyzed. The research gave determined method of air leakage characteristic curve with experimentation and empirical formula. The test method was recommended in practical application. It showed that the leakage rate was influenced by air tightness and velocity, and the influence of aging was great. There was a maximum of 20 times difference of air leakage index in GB 29753, GB/T 5600 and GB/T 7392. Test pressures of 50Pa,100Pa and 250Pa were corresponded to velocities of 30~40km/h, 45~60km/h and 70~100km/h. And the air leakage was about 1∶2∶4 under the conditions of above constant pressure difference. After driving 100000km and 200000km, the air leakage of refrigerated transport unit was increased by 40% and 110% under 250Pa pressure difference. Combined with the actual situation, it was suggested that 250Pa as the uniform constant pressure difference condition for air leakage test standard, and a system should be established to regularly check the thermal performance of refrigerated transport unit. In addition, the air leakage of refrigerated transport unit was influenced by internal partition form, goods stacking form, temperature, etc. A comprehensive and thorough analysis was needed in the later research. 