This study focuses on the preparation and characterization of high glass transition temperature shape memory polymers (HTSMP) for leak plugging applications. The HTSMP was synthesized through a curing reaction and characterized using FTIR spectroscopy, which confirmed the occurrence of a ring-opening reaction resulting in the formation of hydroxyl groups. The glass transition temperature of HTSMP reached up to 136°C, and it maintained a modulus of over 10 MPa at 200°C. Shape memory performance tests showed that HTSMP samples exhibited a recovery rate ranging from 80% to 95%, with a higher recovery rate observed with increased curing agent content. In sealing performance tests, the addition of HTSMP-1 significantly improved the plugging effect of conventional plugging agents, with a pressure-bearing capacity of up to 9.28 MPa at elevated temperatures. These results demonstrate the potential of HTSMP in sealing applications, particularly for addressing fractured leakage in high-temperature deep wells.