The invention relates to a medium seal type small broadband microstrip to waveguide conversion circuit. The conversion circuit is used for solving the microstrip to waveguide conversion problem of a millimeter wave multichannel transceiver module. According to the circuit, a transmission mode of successively matching and transiting a microstrip-a height reduction waveguide-an output waveguide is adopted, thus realizing conversion of the microstrip to waveguide microwave transmission mode. In a first step, the microstrip probes into a height reduction waveguide chamber from a waveguide broad side in a probe form, the size of the height reduction waveguide is 7.11*1mm, miniaturization of microstrip waveguide conversion is realized, matching transmission of microwave signals is realized through adoption of the matching microstrip and a matching air chamber, moreover, a microstrip medium is covered by a waveguide port, and the periphery of the waveguide port is sintered and sealed, thus sealing the waveguide port. In a second step, the height reduction waveguide is transited to the output waveguide by a 4-step ladder, and the size of the waveguide port is gradually transited from 7.11*1mm to 7.11*2mm, thus realizing conversion from a microstrip transmission circuit to an antenna waveguide interface. Compared with the prior art of microstrip to waveguide conversion, the microstrip to waveguide converter has the advantages of broad band, low loss, miniaturization and seal type.