To solve the problem that the conventional optical time domain reflectometer (OTDR) not only required high-power laser output and long cumulative time but also exhibited low resolution, this paper proposes an OTDR with a high-speed single-photon detector based on InGaAs/InP avalanche photodiode. The working repetition frequency of this single-photon detector was 1 GHz, achieving "quasi-continuous" detection, realizing quick capture of return signal at single-photon level without scanning. Finally, with the emitting pulsed laser of 50 ps pulse width and 10 mW peak power, this OTDR whose event dead zone is less than 1 m, could obtain the longest measuring distance of 50 km with 10 cm accuracy.