Abstract: The experiments of mercury intrusion porosimetry and compressive strength of concrete after high temperature are performed,and the relationship between microstructure and mechanical properties of concrete is investigated.The results show that the compressive strength of concrete decreased with the temperature increased,the microstructure of concrete becomes worse,and the macro porous has evident increase.After realkalization,the compressive strength of concrete is improved to some extent,the microstructureis improved obviously,and the macro porous is decreased significantly.The variation of microstructure in concrete is in consistence with the change of mechanical properties,which draws a conclusion that the realkalization repairing can recover the mechanical property and the durability of concrete after high temperature.