Abstract: The aerodynamic performance of a typical over-under TBCC inlet mode transition model has been studied by wind tunnel experiments, and the CFD methods used have also been verified. In the present work, the influence of three typical factors on the experiment is studied by the CFD method, including the side gap, the leading edge bluntness and the forward step on the inner surface. The research results show that the gap between the splitter and the side plate leads to flow spillage between the high and low speed channels. When the gap width reaches 0.5 mm, the total pressure recovery coefficient of the high speed channel is increased by 2.13%, while the flow coefficient is improved by 2.27%. This already has an impact on the aerodynamic performance evaluation of the inlet so that the gap of the model plate should be less than 0.5 mm.The blunted radius of the leading edge has little influence on the aerodynamic performance of the inlet. For the general machining accuracy (0.3 mm), the inlet performance remains basically unchanged. For the general assembly accuracy (0.5 mm), the forward step has very little influence on the inlet flow coefficient, and the total pressure recovery coefficient of the inlet is decreased by 0.44%, which can satisfy the requirements of aerodynamic performance evaluation of the TBCC inlet.