To alleviate the urban heat island effect and reduce the consumption of electricity and expenditure caused by active cooling devices on hot days, many cities in tropical and subtropical areas emphasize the utilization of urban greening areas in current and future urban planning. We utilized the weather research and forecasting model (WRF) to simulate and study the impact of different greening area rates on the urban microclimate in business, residential, and industrial areas in Wuhan city. Meanwhile, we proposed two efficiency coefficients to evaluate the variable cooling benefit of the improvement of the greening area. The results show that greening areas and water bodies are the cooling sources of cities and that industrial areas benefit the most from improvements in the greening rate, with the average temperature declining by 1.06 °C with a 20% increase in the greening rate, while the corresponding values of residential and industrial areas were 0.98 °C and 0.92 °C, respectively. This research provides a reference for the future planning of tropical and subtropical areas to help improve the urban microclimate, thermal environment, and environmental comfort on hot days.