A high-power frequency doubler at 200 GHz has been designed and implemented based on GaN Schottky diodes. This frequency doubler replaces traditional GaAs Schottky diodes with high-power-capacity Gallium Nitride (GaN) Schottky diodes and combines them with Aluminum Nitride (AlN) substrates, which have high thermal conductivity, significantly enhancing the heat dissipation and output power of the doubler. A suspended microstrip-waveguide transition structure with a wedge-shaped membrane is employed, which realizes mode conversion and codirectional input and output by inserting a wedge-shaped membrane into the standard rectangular waveguide, achieving miniaturization of the frequency doubler. Considering the impact of temperature on diode operation, the traditional diode model is modified, and electro-thermal coupled simulations are performed. Actual test results indicate that under 500 mW continuous-wave input, the doubler outputs more than 20 mW in the frequency range of 190~220 GHz and achieves a maximum output power of 36 mW at 218 GHz, with a conversion efficiency of 7.2%.