The window system is generally regarded as the most vulnerable building system for the indoor energy performance of green buildings. Window systems are given significant attention by architects and engineers, especially in areas with long summer and high solar radiation such as the subtropics. This study aims to develop a standard window-to-floor ratio (WFR) system for green residential buildings in the subtropics. Using Autodesk Revit as the interface, a real high-rise residential building was digitalized and imported into Ecotect for energy consumption analysis. Comparative analyses were conducted to determine the optimal WFR for building energy efficiency. Results demonstrated 0.23 as the optimal WFR in Xiamen, one of the typical subtropical cities in Asia. Furthermore, accompanied by a four-sidefins sunshade device and a double glass window (DGW) with clear “glass+air gap+reflective” glass, the building energy consumption was further reduced by 34.47% compared to the initial model, which successfully met the optimization target of 30%, set according to the green building standard. The results of this study are helpful to architects and building engineers when designing or retrofitting green buildings as we provide specific support for design features for energy performance.