The invention discloses a coherent optical time-domain reflectometer based on detection frequency coding, which comprises a laser, a first coupler, a frequency coder, a radio-frequency driver, a circulator, an optical interface, an optical filter, a second coupler, a photoelectric detector, a radio-frequency amplifier, a data acquisition module, a signal processing module and a display module. Laser light emitted by the laser passes through the frequency coder for frequency coding to obtain pulses of detection frequency pulse light and filling light which is complementary to the time sequence of the detection frequency pulse light. The frequency of the pulse of the detection frequency pulse light is different from the frequency of the pulse of the filling light. After the respective backward Raleigh scattering signals of the detection frequency pulse light and the filling light in an optical fiber line are fed back, the detection frequency pulse light and the filling light enter the optical filter through the third port of the circulator, the filtered detection light signal is coherent with local oscillator light at the second coupler, then coherent medium-frequency signals are output by the photoelectric detector and finally the medium-frequency signals are acquired and processed to obtain a time-domain curve which reflects the characteristics of the optical fiber line.