We report a study on the triple-shape memory polymer composition of polyurethane/polymethyl methacrylate (PU/PMMA) synthesized using reaction-induced phase separation. Microscopic phase separation, which forms sea-island structures, enhances the shape memory effect, and phase separation can be achieved by changing the volume ratio of the two components in the polymer composites. In this study, we used the same method and synthesized 5 composites with different volume ratios of PU/PMMA. The composites with a PU/PMMA ratio of 2 : 1 exhibited triple-shape memory effect. The PU/PMMA composites were characterized by X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), scanning electron microscopy (SEM) and dynamic mechanical analysis (DMA). Based on the studies, we propose that reaction-induced phase separation can be used as a general approach to achieve the shape memory effect.