The dielectric surface breakdown has become a major factor of restricting the output power of high power microwave source. And the multipactor is the key phase of the process of the dielectric surface breakdown. In this paper, the multipactors in external magnetic fields with different strengths are simulated by using the particle-in-cell method. And the effect of the external magnetic field on multipactor is also studied. The results show that when the external magnetic field reaches a certain value, the multipactor is suppressed in the half time of the microwave transmission. So the power capability can be increased to more than four times through the suppression of the multipactor by the external magnetic field under the ideal condition.