A downstream wind turbine experiences a decrease in power output owing to the wind speed weakening caused by the shelter of tower. In order to calculate quantitatively the effect on WTG's output power caused by the shelter of tower, a wind speed variation model of tower was proposed. The presented model was used to study the effect of the flow around the tower on WTG's output power, which takes into account the wake effect, mountain terrain, and random wind direction. The output power of wind turbine was calculated by using simulation method with consideration of the flow around the tower and the results were compared with the actual monitoring data of a wind farm in Shanxi. Results show that the influence of flow around the tower on WTG cannot be ignored in some specific conditions. Therefore, in order to provide a theoretical guidance for micro site selection of wind turbine, it is necessary to evaluate the influence of the flow around the tower on power output model of WTG.