HgCdTe avalanche photodiodes （APDs） of linear mode have attracted much attention for their prospective applications, due to the lowest excess noise close to zero, operating bias in the range of -12~0V, and linear gain tunable as well. In this paper, we investigated the dark current mechanism and Okuto-Crowell model based on the PIN structure of mid-wave infrared HgCdTe e-APD. The characteristics of dark current and the gain were simulated. The thickness and carriers concentration of intrinsic region were discussed, which would impact on the dark current and the gain. The results indicated that the peak intensity of electric field would significantly increase the band to band generation, even up to several orders of magnitude. BBT current could be suppressed by increasing the thickness of the intrinsic region, as well as reducing the carrier concentration. Unfortunately, the gain and BBT current would both increase with electric field. The gain normalized dark current （GNDC） is prospective by means of tradeoff between the dark current and the gain. In our studies, intrinsic region with 3 μm and the concentration less than 5×10¹⁴cm^-3 are optimal. The APD I-V results demonstrated that the theoretical model was identical with the experimental data.