Herein, we use an optical phase locked loop (OPLL) for locking the local laser's phase to achieve better laser phase-lock and obtain highly coherent unit beams. First, the cross-correlation function of the phase-locked laser's complex amplitude is determined, and the coherence between different phase-locked lasers is evaluated with a cross-correlation function. The influences of several key parameters, such as the linewidth of the reference light, time constant of the OPLL system, and the difference values of the local laser's linewidth on the function's value are analyzed. The evaluation parameter M is used to characterize the influence of the difference values of the local laser's linewidth on the coherence of the phase-locked laser keeping other parameters constant. Furthermore, the influence of the difference values of the local laser's linewidth on the phase-locking effect is analyzed in detail, and the related formula for calculating a reasonable value range of the related parameter is obtained. Our results indicate that the time constant of the OPLL system must be designed on the basis of the linewidth interval of a local laser to achieve effective phase-locking.