The frequency modulation phase response of single section distributed feedback (DFB) semiconductor laser has a 180° phase reversal in 0.1-5 MHz, which makes it difficult for phase locking of optical phase-locked loop (OPLL) when single section DFB semiconductor lasers act as slave lasers. To solve the problem of phase locking in single section DFB semiconductor lasers, we add the adjustable lead phase shift functional circuit and adjustable gain function unit in the single feedback loop, and optimize parameters of phase shift to accomplish phase locking of DFB laser with MHz level linewidth. The residual phase noise of improved OPLL is 0.012 rad2, and the linewidth of DFB laser is compressed from 2 MHz in free-running state to 10 kHz in phase-locked state. In addition, we research the influence of loop gain of the feedback loop on the phase locking performance of OPLL and give the optimized loop gain parameter.