The overall benefit of power generation for wind farm can be maximum by scientifically and rationally incorporating small wind turbines in the wind farm already built. The technical feasibility analysis of adding small wind turbines is made according to the existing practice of wind turbines in parallel operation. This paper studies the effect of wake effect between wind turbines in complicated terrain on output power of wind farm. The improved Lissaman model is developed considering the occlusion wake effect. The relationship between wind speed and wind turbine output with different tower heights is established. The model is universal and can give any optimization scheme of increasing wind turbines in the wind farm. Example simulation results show that the overall benefit of power generation for wind farm is expected to increase. This work is supported by Shanghai Science and Technology Innovation Action Plan(No. 16DZ1203504).