In order to study the characteristics of water migration in cement mortar after high temperature, the surface of cement mortar samples was coated with wax (leaving an upper surface) after being treated at 300, 400, 500 ℃. Then the non-waxed end of samples was placed 2 cm below the liquid surface for self-imbibition test. The water migration state and characteristics of cement mortar samples after high temperature were studied by nuclear magnetic resonance (NMR) technology T2 spectrum and NMR imaging. The results show that with the increase of treatment temperature, the proportion of gel pores and air-voids or cracks in samples increases, while the proportion of capillary pores gradually decreases, and the water absorption mass of samples decreases. The water absorption mass increases rapidly in the first 24 h, then slows down and tends to be stable. The high temperature affects the capillary water absorption. When the treatment temperature is less than or equal to 400 ℃, the high temperature could increase the capillary water absorption. When the treatment temperature is greater than 400 ℃, the capillary water absorption is weakened. By introducing the capillary absorption coefficient S to characterize the capillary water absorption, S is greater than 10 before 6 h and decreases by more than two orders of magnitude after 6 h. There is a preferential pore size for water migration. The preferential pore size for migration in capillary pores is 10~480 nm, and the preferential pore size for pores or cracks is 1 680~16 800 nm. With the increase of immersion time, water tends to transfer from large pores to smaller pores. Finally, the migration position of water can be observed in real time from outside to inside by NMR imaging, and the migration speed is positively correlated with the temperature of samples.