The beam quality of the accelerator directly affects its application performance, and the structure of the electron gun determines the initial beam state of the accelerator. Optimizing the RF grid-controlled electron gun suitable for rhodotron and reducing its emittance can obtain higher CT imaging resolution. The Particle Tracking solver based on CST is simulated and analyzed to obtain a beam with a normalized rms emittance of 0.4918 mm·mrad and a peak current of 35 mA. The influence of changes in electron gun structure on the emittance of and TWISS parameters is also discussed. The results show that when the focus angle changes, the normalized rms emittance is greatly affected. And at 67.5°, 60°, and 45°, the minimum normalized rms emittance is 0.2617 mm·mrad when the focus angle is 60°.