Using the ABCD transmission matrix and Collins diffraction integral formula and by converting the relation between the beam and cavity coordinates, we derive an analytical expression for the cross-section distribution of a light field at different angles of incidence in the cavity of a fusion device after a flat-top Gaussian beam passes through a lens. This analysis focuses on the effect of angle of incidence of the beam on the light-intensity distribution at the focal point in the cavity. Simulation results show that as the angle of incidence of the beam increased, the light intensity of the spot at the focal point is affected in the X-axis direction, and the light spot diffused along the X-axis direction, but the light-intensity distribution in the Y-axis direction barely changed. As the transmission distance along the optical axis increased, the spot diameter in the cross section of the target cavity became larger, uniformity became worse, and spot position deviated to the right. Conclusively, the angle of incidence and distance of the beam transmission must be appropriately selected to obtain a light spot with good uniformity in the cavity.