Passive phase locking of two fiber lasers which were coupled by a common ring cavity had been demonstrated, and the effect of coupling strength between them on the phase locking performance had been investigated in detail. The ring coupled cavity was chiefly composed of two 2×2 fiber couplers, which provided a common channel for mutual injection coupling and made the output phases of component lasers were primarily synchronized. The theoretical model of analyzing the effect of coupling strength was presented, which was based on the fact that the coupling strength between the fiber lasers was mainly determined by the coupling ratios of fiber couplers, and the effect of magnitude and difference of these coupling ratios on the circulating intensity in the common ring cavity and the output intensity of the phase locking array were investigated in theory. Moreover, efficient phase locking of two Erbium-doped fiber lasers with the common ring coupled cavity had been demonstrated, and the coherence of output lasers and their combining efficiency were also studied by analyzing the far-field interference pattern and output power of the array in experiment. The research results indicatd that adequate coupling strength was a necessary condition to achieve effective phase locking, and improving the coupling strength by increasing the coupling ratio can enhance the coherence of the phase locking array and decrease the combining efficiency slightly.