In coherent LiDAR remote ranging, in order to improve the detection range and range resolution of the LiDAR, a large time broadband product signal such as a frequency modulation signal is usually used to modulate the optical carrier, and pulse compression processing is performed at the receiving end. To reduce the data required for digital signal processing at the receiving end and improve the real-time performance of the calculation, it is necessary to down-convert the received signal to an appropriate frequency band. The traditional heterodyne radar receiver requires a single down conversion of the optical and electrical signals, resulting in a more complex system structure and by the limitations of the device non-idealisation, additional noise is introduced during the downconversion process. In addition, there is the problem of image frequency noise interference, which leads to the degradation of the SNR of the demodulated signal. In this paper, a scheme is proposed to downconvert the optical signals to the frequency band required for pulse compression, which is carried out using orthogonal demodulation to simplify the system structure and suppress mirror frequency noise. Firstly, the local oscillator light is frequency shifted and divided into two beams, and the two local oscillators are made orthogonal to each other by controlling the phase. The signal light is divided into two beams and mixed with two local oscillators on the surface of the photodetector. Then, the electrical signals are collected and the amplitude-phase imbalance is corrected through relevant algorithms. Through simulation and experiments, this method can effectively simplify the structure of the laser coherent radar receiver system while avoiding mirror frequency noise interference. At a sampling rate of 10 GSps, the SNR of demodulated signal is improved by about 3 dB compared to a heterodyne receiver.