Abstract: At high temperature, the microstructure of strain hardening cementitious composites (SHCC) was damaged, which leaded to the deterioration of mechanical properties, impermeability and microstructure. The evolution of mechanical properties and capillary water absorption law of SHCC specimens with different fiber contents exposed to 20℃ (normal temperature), 105℃, 200℃, 400℃, 600℃ and 800℃ were studied, and the degradation mechanism of macroscopic properties of materials was analyzed from the perspective of microstructure by using low-field nuclear magnetic technology. The results show that when the heating temperature increases from 20℃ to 105℃, the dynamic elastic modulus decreases, but the compressive strength and flexural strength increase. When the heating temperature rises to 200℃, the compressive strength and dynamic elastic modulus of SHCC change little, but when the temperature is higher than 400℃, both of them decrease rapidly. When the heating temperature is increased from 105℃ to 200℃, the flexural strength of SHCC specimen decreases significantly, and when the heating temperature is higher than 400℃, the flexural strength further deteriorates. The fiber content has no obvious regular effect on the residual mechanical properties of SHCC specimen after high temperature. In addition, when the heating temperature is lower than 200℃, SHCC has poor capillary water absorption performance and has a certain capillary infiltration resistance. Above 400℃, the high temperature damage can significantly promote the capillary water absorption rate and the water absorption capacity of SHCC specimen. When the temperature is lower than 200℃, the initial capillary water absorption coefficient of SHCC specimen with higher fiber content increases more rapidly, and the capillary water absorption capacity is stronger. The microstructure of SHCC specimen is relatively dense when the heating temperature is lower than 200℃. When the temperature exceeds 400℃, the melting of fibers inside the SHCC specimen and the generation and propagation of cacks lead to significant deterioration of mechanical properties and improvement of capillary water absorption. In the meantime, the volume fraction of cracks in SHCC specimens increases with the increase of fiber content after high temperature.