Abstract: The numerical simulation of the high altitude simulation test for a liquid hydrogen/liquid oxygen rocket engine was carried out by using FLUENT software, and the results were compared with the measurement data of the test. Visual C#.NET software and FLUENT software were used to calculate the ejection parameters of the secondary throat diffuser, and their numerical calculation results are compared. The flow field of the secondary throat diffuser was obtained by using FLUENT software under various aspect ratios, or contraction ratios or entrance lengths (Xst), and the influence on the engine startup was analyzed. The results show that the numerical simulation method is reliable.