In this paper, the analytical expression of the light field distribution in the cavity and the wall of the laser indirect driven fusion target after the flat-top Gaussian beam is focused by the finite number of small amplitude modulated defects are derived. The influence of the size of the defect and the modulation amplitude on the intensity distribution is studied in detail. The influence of incident angle on the diffracted light field of the modulated beam projected onto the cavity wall is further studied. The results show that the larger the size of the defect, the larger the modulation amplitude of the defect, the more obvious the disturbance of the beam, and the longer the transmission distance required for the optical field distribution to be restored to the flat-top Gaussian distribution. When the incident angle of the incident beam is less than 10°, the path of the beam before reaching the cavity wall is long, and the spot projected by the modulated flat-top Gaussian beam on the cavity wall diffuses to the x-axis direction. The change of intensity disturbance caused by defect is obviously weakened, and the spot intensity decreases as a whole.