This paper proposes a new approach for calculating mechanical power of wind turbine in the detailed model for mountainous wind farm employing variation coefficient as non-uniformity index of wind speed. At first, a spatial and temporal distribution characteristic table about wind speed in mountainous wind farm is built. Subsequently, the upper percentile and lower percentile would be acquired according to the variation coefficient and its confidence level from the characteristic table. By this means, a wind speed domain with the maximum and minimum values can be formed. Consequently, the corresponding domain for mechanical power of wind turbine in mountainous wind farm could be obtained. The dynamic simulation study has been carried on a high-altitude mountainous wind farm in southwest region of China using PSS/E. It includes two working conditions near cut-in wind speed and average wind speed. The results verify the effectiveness of the proposed method comparing with the traditional method using average wind speed, and also prove that it is reasonable for taking variation coefficient as a non-uniformity index of wind speed. Furthermore, some discussions would be useful for dynamic characteristics analysis considering non-uniformity and uncertainty of high-density wind power integrated power system in mountain areas.