For 3 kW diode laser used in robot surface modification system, the focal spot is uneven. A kind of focus and homogenization method by using the Fresnel lens for the focusing system of high power diode laser has been put forward, and a uniform rectangular spot of 10 mm×2 mm has been designed. Through the simulation analysis of Zemax and Matlab, the influences of the pitch of Fresnel lens and the divergence angle of incident rays on the homogeneity of the focal spot have been studied. The results show that the best uniformity of the focal spot is about 94.90% when the pitch of Fresnel lens is less than 1mm and the uniformity gets worse and worse with the increase of the pitch of Fresnel lens. As the pitch increases to 2.5 mm, it has less influence on the uniformity of the focal spot, and the uniformity basically stables at around 93.85%. Meanwhile, the uniformity will be improved with the increase of divergence angle, but large divergence angle will increase the difficulty of focusing and make the uniformity get worse and worse. When the pitch of Fresnel lens is 1mm and the divergent angle is from 12.5 mrad to 20 mrad, the best uniformity of the focal spot is 95.22%.