The traditional Optical Time Domain Reflectometry (OTDR) technology uses back Rayleigh scattering power for fiber detection. In transmission lines with optical amplifier, Erbium-Doped Fiber Amplifiers (EDFA) are mostly used to compensate for the loss of signal light and extend the transmission distance of the signal light. Since the EDFA amplifies the signal, it will generate spontaneous radiation noise, and the continuous accumulation of noise will rapidly decrease the signal-to-noise ratio measured by the OTDR. The paper mainly expounds the principle of Coherent Optical Time Domain Reflectometry (C-OTDR) technology. The detected signal optical power is concentrated to the intermediate frequency by the coherent detection method, and most of the noise can be filtered out by band-pass filtering the frequency signal. It can continuously extend the detection distance through the EDFA. We also propose to use the frequency shift keying method to eliminate the optical surge problem caused by the cascade of multi-stage repeaters in long-distance transmission of submarine cables. Through the development of self-developed coherent optical time domain reflectmeter, the actual test of C-OTDR technology in the simulated submarine cable multi-span transmission system has reached 3 000 km, which fully meets practical conditions.