A numerical model was established by the level set method to simulate the etching profile evolution in HgCdTe ion beam etching. The input parameters are: mask thickness、the slope of mask sidewall、trench width、ion angular distributions, etching speed, et al. Etching lag and etching profile of HgCdTe were simulated and compared with experimental results. The results shows that, given nominal trench width 4?10?m, the errors between simulated etch depths and that of experiments are 6~20%. The profile evolution of etching mask was simulated and an example was given to illustrate how to design the mask thickness to improve the aspect ratio.