The diffractive optical element (DOE) is widely used to generate various illumination pupils in the deep ultraviolet (DUV) projection lithography system. According to the specifications of DUV lithography machines, the designed DOE should possess high diffraction efficiency and high uniformity. Traditional algorithms such as Gerchberg- Saxton (GS) algorithm (GSA) and modified GSA get high diffraction efficiency with low non-uniformity. Global searching methods such as gene algorithm and simulated annealing algorithm need huge computing effort to receive high diffraction efficiency and high uniformity. To address these issues, a mixed gradient algorithm based on GSA is proposed. It can accelerate the rate of convergence and improve the signal to noise ratio simultaneously, and the phase retrieval image is improved. The DOEs generating conventional, quadrupole and customized illumination modes are designed and analyzed based on the proposed method, then the phase is quantized with 16 levels. The diffraction efficiency will reach 92%, and the non- uniformities of the proposed algorithm can reach 3.98% and 2.3% for the conventional and quadrupole illumination modes, respectively. The received efficiency for customized DOE is 91% , and its pattern recovery error is 5.8%. It provides a practical method of DOE design in projection lithography system.