A millimeter power synthesizer based on a novel suspended microstrip transition structure has been proposed. The terahertz power combiner combines/distributes power through a traditional waveguide power splitter with one to eight channels, and connects the eight channels of power splitters with a new suspended microstrip transition structure. The proposed new microstrip transition structure is easy to process, and by optimizing the length of the suspended microstrip substrate, the two sides of the substrate are mounted in the protruding waveguide sidewalls, which can suspend and fix the entire microstrip inside the waveguide without affecting transmission performance. The average insertion loss test values of the processed samples remained stable below 0.5 dB. Finally, the overall optimization analysis was conducted on the power combiner. Simulation results showed that within the operating frequency range of 180~220 GHz, the S11 parameter of the power combiner remained stable below -15 dB, and the loss from the input port to the output port of the power combiner was approximately 0.25 dB.