In this paper, an S band all cavity extraction relativistic magnetron with permanent magnet is theoretically designed and numerically simulated, and an experimental investigation is also carried out. The initial structural parameters of the relativistic magnetron are obtained through theoretical analysis, and the model is optimized by particle simulation. The permanent magnetic field generation structure is designed according to the needs of the guiding magnetic field. Simulation results reveal that this permanent magnet relativistic magnetron could generate a microwave power of 1.978 GW corresponding to a power conversion efficiency of 49.2% when the driver voltage is 500 kV. This permanent magnet tube is tested on the high-voltage pulse drive platform. In the experiment, gigawatt class output microwave power was obtained, corresponding to a power conversion efficiency of about 40%. The experimental results are in good agreement with the simulation results.