Abstract: During the operation of high-speed trains, the condensing air flux of the air-conditioning system can decrease sharply, which leads to the cooling efficiency decreasing of the air-conditioning system, and sometimes even the system shutdown due to the overheating issue. In this study, the condensing air flux under different deflector heights were investigated using the three-dimensional unsteady incompressible Reynolds-Averaged Navier-Stokes equations together with the k-ε turbulence model and the DDES method. In the meantime, the resistance variation under different deflector heights were analyzed. The results show that the deflectors installed outside the condensing outlet have a significant effect on alleviating the rapid decrease of condensing air flux with the increase of the train speed. By changing the flow structure outside the condensing outlet, the deflector induces high-speed flow on the side of the train to form an area of negative pressure so as to alleviate the decrease of the air flux. The height of the deflector directly determines the condensing air flux, with higher deflector generating larger condensing air flux but also higher resistance. Overall, a 45 mm high deflector can effectively improve the condensation air flux decreasing situation while induce relatively small amount of resistance.