Phase change material (PCM) has been widely integrated in building envelops to increase their thermal inertia performance. To evaluate the thermal inertia performance of materials and envelops, Chinese Thermal Design Code has provided three indicators, namely, thermal storage coefficient, thermal resistance and thermal inertia index. The existing simplified method calculating the thermal storage coefficient is only applicable for materials with constant thermal properties. For those with varying thermal properties, such as PCM, however, further developments are still required. To solve this issue, both dimensional analysis and numerical simulation were carried out to develop relationships between the thermal storage coefficient of PCM and its other thermal properties (e.g. thermal conductivity, density and the effective equivalent specific heat). Based on the developed relationships, a simplified method calculating the thermal storage coefficient of PCM was proposed in this study. This simplified method was then combined into the thermal inertia index for evaluating the thermal inertia performance of building envelops with PCM.