In order to reduce the heat transfer between indoor and outdoor environments and reduce the influence of outdoor ambient temperature fluctuation on the indoor environment, adding micro-PCM to the building envelope is one of the effective means. Micro-PCM can "intelligently" control temperature by absorbing or releasing heat during phase change to maintain temperature stability. In this paper, the phase change temperature-adjustable mortar was prepared by using micro-PCM with a phase change temperature of 25°C, and was painted on the surface of cement wallboard (300 × 100 mm) to form a phase change thermostatic mortar layer with a cross-section size of 300 × 20 mm. The solar radiation on the outer surface of the building envelope was simulated by an incandescent lamp. The influence of micro-PCM on the temperature control performance of cement wallboard was tested. The thermal performance of the PCM wallboard was simulated by COMSOL software. Results show that compared with ordinary cement wallboard, when the micro-PCM content is 40%, the maximum temperature of the inner wall can be reduced by 5.17°C, and the time to reach the maximum temperature is delayed by 145 minutes. The temperature fluctuation amplitude of the inner wall is reduced by 1.90°C, and the maximum instantaneous heat transfer is reduced by 22.202 W/m2.