Abstract: As the mode competition of magnetrons becomes more intense, to better study the mode of magnetron, a mode decomposition method is proposed in this paper, and the A6 relativistic magnetron is selected as the research object. The results of mode decomposition show that when the relativistic magnetron oscillates, there will be a phenomenon of multi-mode coexistence, and the magnetron will oscillate at the frequency of the eigenmode with the strongest field strength. At the same time, the results show that the degenerate mode coexists and oscillates in the same trend. Combined with PIC simulations, it can be inferred that the relativistic magnetron with radial output structure can operate at the degenerate mode and obtain stable output signal.