In order to meet the high uniformity and different illumination mode requirement of the mask plane of the immersion lithography illumination system, the illumination mode converter system was studied. Diffractive Optical Elements (DOE) were used to generate various illumination modes, started from grating structure, the process of converting the grating into DOE by two-step transformation was analyzed. And a discrete sampling encryption algorithm was proposed. By examples of quadrupole illumination DOE with sampling width 1-5 μm, the correspondence relationship between the DOE feature size, diffraction efficiency and the intensity distribution non-uniformity was disclosed. The design results show that as the sampling line width decreases, the diffraction efficiency and uniformity of the shaped beam will be greatly improved. The 16-step illumination mode conversion DOE with feature size of 1.76 ?滋m×1.76 ?滋m was fabricated by contact lithography.The optical non-uniformity and diffraction efficiency of DOE under different illumination modes were tested by setting up an optical test platform. The results meet the design requirements and verify that the discrete sampling encryption algorithm can effectively guide the design of the DOE for the illumination mode transformation system.