The invention provides a coherent optical time domain reflectometer based on a phase conjugation double wavelets. The reflectometer comprises a laser, a 1*2 optical coupler, an acoustic optical modulator, an optical circulator, an OTDR data processing module, a polarization multiplexing modulator and a polarization multiplexing coherent detector. The polarization multiplexing modulator is used for modulating two paths of phase conjugation COTDR detection electric signals on detection optical pulses and carrying out polarization beam combination on two paths of orthogonal modulated information so as to form detection optical signals transmitted on optical fibers. The polarization multiplexing detector is used for decomposing optical signals from detection light input ends into two paths of orthogonal signals, and mixing the orthogonal signals with local oscillation light from local oscillation input ends so as to carry out coherent detection. At last, the two paths of optical signals after the coherent detection are superposed and then output. According to the invention, the dynamic range of the coherent optical time domain reflectometer (COTDR) is expanded and the coherent optical time domain reflectometer is suitable for line detection of long distance optical fiber transmission.