By means of numerical calculation, the propagation characteristics of the open and slowly varying section resonator of 140 GHz and TE22,6 mode gyrotron are simulated; the resonant frequency and quality factor of the resonator are calculated. The CST software is utilized to simulate the high-frequency resonator. The electric field distribution cloud diagram of the cross-section inside the cavity is obtained. The data obtained through the experiment and the simulation software are compared and the two are in good agreement. The test results show that when the magnetic field is 5.48 T, the current is 28 A and the voltage is 68.6 kV, the gyrotron could work in TE22,6 mode, and the average output power is 0.25 kW, the peak power is 0.56 MW; when the magnetic field is 5.68 T, the electronic current is 27.6 A and the electronic voltage is 69.12 kV, the gyrotron could also work in TE22,6,2 mode, and the average output power is 0.21 kW, the peak power is 0.47 MW.