Ultraviolet lithography is one of the core technologies in micro nano processing. Understanding and controlling the diffraction effects during the lithography process can effectively improve the quality of lithography. This paper systematically studies the influence of different contact modes and Fresnel diffraction on the lithography results in the application scenarios of commercial contact ultraviolet lithography machines. The experiment compared the exposure results of different contact modes and found that Fresnel diffraction would cause pattern distortion in soft contact and separation modes. In addition, we also observed typical Poisson bright spots in the lithography results. Therefore, this paper establishes a theoretical model for photolithography diffraction, and the Matlab simulation results obtained based on this model are highly consistent with the experimental results. The experimental results indicate that the vacuum contact mode can effectively suppress diffraction and ensure accurate pattern transfer. The controllable diffraction effect in the separation mode can provide new ideas for the preparation of submicron structures, and the photolithography diffraction simulation method proposed in this paper provides a working basis for this.