Adapting to the local climate is the key to developing nearly-zero energy buildings (NZEBs). During cooling season in Western China, the climate conditions are characterized by a large daily temperature range and high solar radiation, and improving the thermal storage performance of buildings is an effective passive cooling design strategy for NZEBs. This study aims to investigate the thermal storage performance of building envelopes under free-running conditions. A phase change material (PCM) is integrated in an experimental wallboard to enhance its thermal storage performance. To simulate the thermal environment in free-running buildings, a double-sided periodic thermal effect is applied as the indoor and outdoor conditions during the experiment. Different orders of the material layers and thicknesses of the PCM layer are compared to investigate the key factors that affected the thermal storage performance. The thermal inertia index and coefficient of heat accumulation are employed as key parameters to quantify the two factors. The results indicated that enhancing the coefficient of heat accumulation for the wallboard by optimizing the material layer order can reduce the fluctuations in the indoor air temperature by 31%. However, only increasing the thickness of the heat storage material does not significantly improve the attenuation and time delay effect of the wallboards. The coefficient of heat accumulation is proposed as a key parameter for NZEBs design. The design prototype and key parameters of the thermal storage wall considered in this study may provide a reference for the development of NZEBs in Western China.