To expand the application of magnesium phosphate cement (MPC) in high-temperature environments, this study investigates the performance and mechanisms of limestone powder blended magnesium phosphate cement (LP-MPC) after exposure to high temperatures. The results indicate that as the temperature increases, the mass loss rate of LP-MPC gradually rises, while the compressive strength shows a trend of initially decreasing and then stabilizing, accompanied by a deterioration in pore structure characteristics. Below 600°C, dehydration of MgKPO4·6H2O predominates, and the addition of LP is beneficial for enhancing the compressive strength of MPC. However, above 600°C, the thermal decomposition of CaCO3 becomes significant, leading to an increased mass loss rate with higher LP content and a decrease in compressive strength as LP content increases. The effects of temperature and LP content on the deterioration of the pore structure are pronounced. After high-temperature exposure, the pore structure of LP-MPC with in the same fractal region becomes less complex as the temperature rises. The correlation between the fractal dimensions of harmful pores and multi-hazard pores with compressive strength is stronger.