The composition gradient was introduced into the structure design of HgCdTe detector, and a method was proposed to reduce the thermally excited carrier concentration near PN junction. The model of dark current mechanism was established, and the analysis of dark current components at high temperature showed that reducing the influence of the thermally excited carrier concentration on the junction was the key to improve the operating temperature of the detector. Different electric fields were built near the PN junction by using the composition gradient. The curves of dark current and noise of the sample with temperature under different electric fields show that the stronger the electric field is, the more obvious the effect of reducing the thermally excited carrier concentration near the junction. Based on the data analysis, it is proposed that the built-in electric field generated by the composition gradient of 10³ V/cm can inhibit the diffusion movement of thermally excited carriers to the junction region, which effectively reduce the concentration of thermally excited carriers near the junction region.