For compact high-power microwave devices operating at low magnetic field, a compact S-band relativistic magnetron operating at low magnetic field was designed and simulated with three-dimensional particle-in-cell codes. This tube radiates TE₁₁ mode in circular waveguide with diffraction output structure. As the cutoff radius of TE₁₁ mode is the smallest in circular waveguide, compared with higher modes, the radius of the output waveguide could be reduced obviously. The output performance as a function of magnetic field, radius of waveguide and angle was studied. Typical simulation results show that microwave power of 567 MW was generated at 2.37 GHz when the voltage and magnetic field were 352 kV and 0.34 T, the power conversion efficiency was 62.5%, and the radius of waveguide was only 77.5 mm.