Polyurethane microcapsules containing phase change materials (microPCMs) of n-octadecane applied in building materials were successfully synthesized by an interfacial polymerization in aqueous styrene-maleic anhydride (SMA) dispersion with diethylene triamine (DETA) as a chain extender reacting with toluene-2, 4-diisocyanate (TDI). FTIR and SEM morphologies results confirmed that the shell of microcapsules was polyurethane. Thermal stability and heat absorption simulation were investigated by TGA, DSC and an environmental simulation apparatus. TGA data showed that the decomposition of the microcapsules began at approximately 339oC at 2oC/min of increasing temperature and 320oC under 20% humidity, respectively. Shell structure was affected by environmental changes including temperature and humidity. The microcapsules will be compact and service longer time in practical application under gentle environmental changes. Also, the thermal absorption characterization was performed on a self-made design to improve the understanding of the thermal properties of dried microcapsules in practical application.