As diffusion control caused by the thermal annealing of in-situ As-doped HgCdTe grown by molecular beam epitaxy (MBE) was studied. HgCdTe with controllable As diffusion length is obtained at a lower annealing temperature, which is easy to form a PN junction profile that meets the design parameters. It provides a basis for the subsequent development of new HgCdTe FPA devices. It is found that the longitudinal distribution of As concentration of the in-situ As-doped HgCdTe changed under different Hg pressures during the thermal annealing process. And through theoretical calculations, As diffusion coefficients under different Hg pressures are obtained. Meanwhile, the dark current simulation of HgCdTe P-on-N structure with different As diffusion lengths was carried out through numerical simulation, which verified the importance of deep-advancing process for As-doped HgCdTe PN junction.