A heterodyne all-fiber optical phase-locked loop is designed to suppress the relative frequency noise of two semiconductor lasers in passive resonator fiber gyros. The relationship between the bias stability of the passive resonator gyros and parameters of the optical phase-locked loop is established and analyzed. Then the mathematical model of the heterodyne digital optical phase-locked loop is built and simulated. Finally, the performance of the designed system is validated with experiments. Which generate a remarkable meaning of the multi-laser system used for all-fiber passive resonator gyros. The frequency difference between the two lasers is locked to the frequency of the reference source, and the deviation amplitude between the beat frequency of the lasers and the resonance frequency of the fiber ring resonator is successfully reduced by more than 15 dB.