This study introduces a new method for assessing the outdoor ventilation environment in Taiwan, with a view to reducing the heat island phenomenon. The ventilation environment is important in cooling the outdoor environment, but its evaluation is very difficult and complex. Therefore, this study develops a simple method for assessing the outdoor ventilation environment, it is used in computational fluid dynamics (CFD) simulation software, only to generate numerical models using empirical building types include rectangular shape and courtyard building, then calculate their wind shadow areas (WSa). Then, many factors, such as the site plan, the size of building and local wind data are used to draw these areas to the scope of WSa. The results of the simple assessment method are verified by performing CFD simulations to corroborate the correlation between simplified assessment method and CFD simulation result, an R2 value of 0.87 is obtained. The method is helpful in improving design decisions in the planning and design stage of site plan.