This paper explores the generation and distribution, the influences on electron motion and interactions of the self-magnetic field in the relativistic magnetron. It is found that the electron beams located close to the accelerator are more obviously influenced by the self magnetic field, leading to the speeding in the angular motion of electron beams, and the mode competition intensifies. Meanwhile, if the cathode lays closer to the accelerator, especially in the region adjacent to the 1/4 length of anode, the mode competition will be further intensified.