Abstract: In this article, the electron trajectory and thermal distribution in the collector region of W-band gyrotron traveling-wave tube were calculated with PIC code and thermal analysis code. Through the optimization of collector magnetic field parameters, the electron distribution in the collector region was simulated and the area of electron trajectory was increased from 17.05 cm2 to 28.52 cm2 (nearly 67% larger), which reduced the power density per unit area. Then the power density was determined to be 1 kW/cm2 and the rate of cooling water flow 1.12 L/s by the thermal analysis of thermal distribution in the collector, which makes the temperature of the inner wall of collector lower than the melting point of the material, us physical cracks will not occur, which could ensure the stability of the collector, so that the whole gyro-TWT could work steadily. Thermal analysis has verified the feasibility of optimal method, and the method has been used in practical tubes.