Abstract: The experiment of microstructure SEM and compressive strength of HPC after exposure to hightemperatures was performed and the relationship between microstructure and mechanical properties of HPC was investigated. The results show that the variation of microstructure in concrete is in conformity with the change in mechanical properties. With the temperature increased, concrete aggregates change from compact to loose, the microcrack is increased and interconnected in quantity and width, and the mechanical properties are decreased accordingly. The vapor pressure of inner-concrete is decreased due to interconnected pores after polypropylene fiber melted, which can alleviate the damage of high temperature to concrete.