The electrochromic (EC) smart window is a promising intelligent technology for improving the energy performance of a building. When applying a new glazing material, the window-wall ratio (WWR) is regarded as a significant factor related to daylight and building energy losses. However, there have not been any studies examining the relationship between the efficiency of the EC smart window and the WWR. The aim of this study is to provide strategies for mitigating energy losses and environmental impacts from window elements in an office building. It focuses on optimizing the WWR of EC smart windows to increase building energy efficiency. Double EC, low-emissivity (low-E), and clear glazing materials are simulated combined with five WWRs from 0% to 80% with an interval of 20%, four window orientations, and humid subtropical and Mediterranean climates. The results show that a 40% WWR in a southern-facing building façade using a humid subtropical climate setting provides the optimum energy performance of EC glazing among all the configurations tested. Since it was also found that EC glazing reduces the cooling load, it is more suitable for climate conditions characterized by a hotter temperature with greater solar irradiation. Future research should consider the impact of WWR when balancing thermal, lighting and ventilation requirements, and the properties of EC glazing.