We propose a new scheme named peak count rate scanning measurement to measure the time of flight by scanning peak count rate with single photon detection and introduce th e operating principle of the scheme.We set up the mathematical model among the number of op tical pulses,the scanning step of gate signal and the length of optical fiber according to the pr ocess of photon counting.We analyze the relationship between the correct probabili ty of peak count rate and optical pulses under given gate signal pulses and scanning step and the result o f theoretical analysis is verified with experimental confirmation using a 50km optical fiber.