Abstract: The illumination uniformity on the capsule surface is very important for inertial confinement fusion (ICF) to achieve ignition. In this paper, we analyze the direct drive illumination uniformity provided by Shenguang Ⅲ prototype laser facility which is designed for indirect drive ICF. Highly uniform irradiation is achieved by optimizing the intensity distribution within a laser beam when the power imbalance and pointing errors are taken into account and the results show that the intensity on the capsule becomes more uniform as laser uncertainties decrease. Furthermore, the relation between optimized intensity on the capsule surface and the laser parameters is analyzed and the results show that we can adjust the mean intensity on the capsule surface by changing optimized parameters with length dimension, which is useful for target design. Finally, the optimized results are applied to direct drive fast ignition and it shows that they are applicable when the cone is in the polar region of the capsule and the half cone angle is less than 30.