Abstract: In order to investigate the reasons for the degradation of dynamic performance of diesel engines under plateau environment, multidimensional simulation under different altitudes was conducted to study the combustion process. A set of characteristic parameters accounting for mixing in combustion chamber was established, basing on it, quantitative analysis of the influence of plateau environment on mixing process was conducted. And then the influence of plateau environment on combustion process was investigated mainly by analyzing the heat release rate in the engine. The results show that, under plateau environment, near the wall of combustion chamber easily forms rich mixture, which leads to a reducing of air utilization rate in the center of combustion chamber, a mass of rich mixture still exists at late combustion process. With the rise of altitude, the center area of high temperature moves to the wall of combustion chamber, and causes the deterioration of combustion efficiency. The influence of plateau environment on premixing combustion is not obvious, however, with the rise of altitude, heat release rate of diffusion combustion reduces gradually, and the duration of combustion increases sharply.