Abstract: In order to investigate the influences of fire fighting water on the mechanical properties of HPC (high porfomance concrete) in a tunnel fire, the variation of the characteristics of samples with fire temperature was researched experimentally, the effect of fire fighting water on the residual compressive strength of HPC was analyzed, and the fitting relationship between the residual compressive strength and fire temperature was given. The research results show that fire fighting water affects the residual compressive strength of HPC greatly. Generally, under a natural cooling condition, the the residual compressive strength of HPC decreases with temperature raising, but goes up slightly at 300 ℃. Under a water injection cooling condition, the residual compressive strength of HPC decreases all the time. Under the same temperature, the residual compressive strength is lower under the water injection cooling condition than under the natural cooling condition, and the greatest residual compressive strength under the natural cooling condition is 1.3 times of that under the water injection cooling condition.