Abstract: An S band high power microwave multiplexer with three channels is investigated through numerical computation. The realized multiplexer works in the band of 2.65~2.75, 2.80~2.90 and 2.95~3.05 GHz. In each pass band, the transmission loss is less than 0.1 dB and the reflection loss is more than 15 dB, the isolation is more than 30 dB. As a result, the energy efficiency is as high as 95%. The power handling capacity of the multiplexer with widened waveguide is demonstrated to be GW level by a full wave analysis software.