The high-efficiency mutual coupling phase locking of magnetron provides an effective technical solution for high-efficiency and high-power arrays based on electric vacuum oscillators. The introduction of the mutual coupling structure makes the mutual coupling magnetron establish a new resonant mode sequence, in which the mode satisfying the high-efficiency phase-locking of the mutual coupling magnetron is the desired phase-locking mode. However, the phase-locked mode is susceptible to the interference of adjacent modes in the mode sequence, resulting in unstable operation. In this paper, a method of regulating the mode distribution by combining the equivalent circuit with the eigenmode analysis is proposed. By regulating the frequency separation of the mode, the phase-locked mode can work in a single mode. At the same time, the matching resonance conditions of the magnetron working mode and the coupling field of the mutual coupling structure are established to realize efficient mutual coupling phase locking of the magnetron. To verify the effectiveness of the method, an efficient mutual coupling model based on S-band MW-level magnetron is designed, and the particle simulation of the phase-locked mode operating characteristics is carried out. The simulation results show that the mutual coupling model can work stably in the high-efficiency phase-locking modes: 0 phase difference mode and π phase difference mode. The locking frequency is about 2.545 GHz, which is close to the free oscillation frequency of the magnetron single tube. The output power of each magnetron is close to the output power of a single tube in free operation. The electronic efficiency is almost the same as that of a single tube, and the mutual coupling phase-locking efficiency reaches 99%, achieving high-efficiency phase-locking.