In digital micromirror device (DMD)-based projection photolithography, the throughput largely depends on the effectiveness of the laser energy utilization, which is directly correlated to the diffraction efficiency of DMD. Here, to optimize the DMD diffraction efficiency and thus the laser energy utilization, we calculate the diffraction efficiencies Ediffraction of DMD with various pitch sizes at wavelengths ranging from 200 nm to 800 nm, using the two-dimensional blazed grating diffraction theory. Specifically, the light incident angle is optimized for 343 nm laser and 7.56 μm pitch-size DMD, and the maximum single-order diffraction efficiency Ediffraction is increased from 40% to 96%. Experimentally, we use the effective energy utilization ηeff = Ediffraction,(m,n)/Σ[Ediffraction,(m,n)] at the entrance pupil plane of the objective to verify the effectiveness of the optimized illumination angle in a lithography illumination system with parallel beams of two wavelengths (343 nm and 515 nm). The ηeff of a “blaze” order at a 34° angle of incidence can be optimized up to 88%. The experimental results are consistent with the tendency of the calculated results, indicating that this optimization model can be used to improve the energy utilization of projection lithography with the arbitrarily designable wavelengths and the DMD’s pitch size.