Abstract: A novel type of microencapsulated phase change materials (microPCMs) with n-Octadecane core shelled by styrene–divinylbenzene copolymer were synthesized by the suspension polymerization. The morphologies and chemical compositions of the microPCMs with four different shell-to-core ratios were analyzed via the scanning electron microscopy and Fourier transform infrared spectroscopy. Based on the measurements from the thermogravimetric analyzers and differential scanning calorimetry, the thermal characteristics of the different microPCMs were analyzed. The results show that the sample microPCMs with 2:1 shell-to-core ratio (microPCM3) has good thermal stability and high heat storage capability, with melting enthalpy and encapsulation efficiency measured at 111.5 ± 0.7 J/g and 51.4 ± 0.7 %, respectively, which are the highest among all the prepared samples. The initial weight loss temperature of the microPCM3 increases to 160.5 ℃ compared to 115.5 ℃ for the pure n-Octadecane. Furthermore, we prepared four different building boards with different microPCM3 contents (0, 10, 20, or 30 wt%) in cement matrix. Compared to the conventional building material, the board with 30 wt% microPCM3 can store 67.82% more heat energy in the typical temperature range of 10–50 °C. We investigated the passive cooling and heating effects of the boards under extreme temperature conditions (10–70 °C), as well as their thermal regulation performance under multiple consecutive thermal cycling experiments. Compared with the cement board without addition of microPCM3, the board with 30 wt% microPCM3 has reduced temperature fluctuation by 59.0% with maximum temperature fluctuation only 23.5 °C. It has shown excellent thermal regulation and thermal insulation properties and is of great application potential in energy-saving buildings.