Aiming at the problem that the air tightness testing index of railway mechanical refrigerated car is lag-ging behind and it is difficult to reflect the air leakage characteristic of the car, the work aims to study the air tightness index of railway mechanical refrigerated car to find the defects in the air tightness design of existing equipment, and then put forward the air tightness index of newly built railway mechanical refrigerated car and the optimum design method of air tightness structure of car body. According to the requirement of GB/T 5600, the air tightness test of B23, B23A and B10 railway mechanical refrigerated cars was carried out in the pressure difference method, and the relationship between running speed, pressure difference and air leakage was analyzed. Through testing and field investigation, the reasons for the large air leakage of existing railway mechanical refrigerated car were grasped, which provided a basis for improvement and optimization. The air leakage of the existing B23, B23A and B10 railway mechanical refrigerated car under 50 Pa pressure difference was 27.0 m3/h, 34.1 m3/h and 26.1 m3/h, respectively. When the pressure difference rose to 125 Pa and 250 Pa, the air leakage of the carriage rose by 90%~320%. The above-mentioned pressure difference conditions roughly corresponded to the running speed of 40 km/h, 65 km/h and 90 km/h of the car. On this basis, the air tightness index of newly built railway mechanical refrigerated car was analyzed and given, and the specific strategy of improvement and optimization was put forward. The air leakage standard of newly built railway mechanical refrigerated car should be set at 50 Pa pressure difference conditions, not more than 10 m3/h, which is 75% less than the existing standard. In addition, through the use of new technology and technology, the structure of carriage, door and drainage outlet is improved, and the leakage points of the car body are reduced, and effectively improves the air tightness of the carriage.