Synthetic aperture lidar has the advantage that the imaging resolution is independent of the detection distance. Compared with microwave, laser has a wavelength at least 3 orders of magnitude smaller. Thus, synthetic aperture lidar can achieve imaging with a higher resolution. However, due to the shorter wavelength, it is also more vulnerable to the phase error introduced by the system itself, which results in imaging blur. To eliminate the influence of this error on imaging, the paper adopts an optical phase-locked loop (OPLL) technology to suppress the random phase noise of the coded synthetic aperture lidar system. At the transmitting end, the laser signal is modulated by an electro-optic modulator driven by the coded signal for detecting the target. At the receiving end, the received echo signal and the local oscillation light are subjected to orthogonal demodulation. The proposed coded synthetic aperture lidar based on OPLL works at the 1550 nm band. Its modulation bandwidth is 1.25 GHz, and the pulse repetition frequency is up to 1.2 MHz/s. The experimental results show that the random phase fluctuation of the system is greater than 100 rad without OPLL. In the presence of OPLL, the phase fluctuation of the internal system is smaller than 0.1 rad. To sum up, the OPLL technology greatly improves the phase stability of the system and the synthetic imaging accuracy in the azimuth direction.