With the rapid development of heavy haul freight train technology in China, especially the new type of freight car brake technology, the difficulty of air brake performance test for long formation trains is also increasing. The structural characteristics and design parameters of each truck brake are the same, and the performance of single vehicle test is the same, but the process of air pressure transmission is different under the condition of initial inflation of train tube and braking relief. By establishing the mathematical model of air pressure in train brake pipeline and combining the single valve and single vehicle physical test, the semi-physical simulation test platform of whole train about brake condition and mitigation condition was established. The hardware-in-the-loop test platform required that the simulation model was controlled in real time through hardware interface, and the model was revised according to the real data collected from the test to ensure the rationality and scientificity of the established test simulation model, and the brake aerodynamics equation was used for analysis and solution. In this paper, the aerodynamic partial differential equations method was used, and the characteristic line method and the gas volumetric charge and discharge model were used for analytical simulation calculation. Combined with the correction function for real-time correction, the real-time control of the semi-physical simulation test system was realized, which provided a theoretical basis for intelligent test platform and an ideal mathematical model.