The stoichiometric ratio of the surface passivation layer of mercury cadmium telluride (HgCdTe) has a crucial influence on the device performance. The influence of Ar+ sputtering energy and incident angle on the stoichiometric ratio of the surface passivation layer is simulated by SRIM software. The simulation results show that in the energy range of 300--500 eV, the nuclear stopping power of CdTe and ZnS for Ar+ is much greater than the electron stopping power, and the nuclear stopping power of ZnS is better than that of CdTe; the maximum sputtering yield of CdTe is found near the ion incident angle of 60°, and the maximum sputtering yield of ZnS is found near the incident angle of 70°; there is a preferential sputtering phenomenon in the sputtering process, and Cd and Zn elements are the preferential sputtering elements. Based on the simulation results, the quality of the passivation layer on the long-wave HgCdTe surface is significantly improved. This method establishes the relationship between the stoichiometric ratio of the passivation layer and the sputtering energy as well as the incident angle, which provides a guiding direction for the actual process. This has certain practical significance for the development of high-performance HgCdTe infrared detectors.