Abstract: The mass loss rate and the relative dynamic elastic modulus of concrete specimens were studied under varying fiber types，temperature and holding time in this work. Studied compressive strength and splitting tensile strength of series of samples which treated under different conditions. Influences of high temperature, holding time and fiber types for the primary physical and mechanical properties of concrete were discussed respectively. The improvement mechanism of organic fiber on the performance of concrete after high temperature was also investigated. The results revealed that, the mechanical performance of plain concrete was declined heavily after high temperature, even burst after temperature arrived 700 ℃ to 800 ℃. Two types of organic fiber addition would significantly reduce the risk of this burst behavior. Mixing organic fiber can reduce the water evaporation and the mass loss of concrete after high temperature. After high temperature treatment, splitting tensile strength of the specimens decreased quicker than the compressive strength. The decrease of strength was obviously enlarged with the increasing of heat holding time. The strength loss after heating would eased by organic fiber, for the in-situ pore structure formed after fiber melted would release a part of stress caused by heating, and prevent the microstructure destruction and explosion of concrete.