Partially coherent light is essential in lithography systems, where it improves illumination homogenization, enhances resolution, and mitigates speckle noise, playing a key role in advanced imaging applications. However, efficiently generating and computing partially coherent beams (PCBs) remains a challenge, particularly in high-precision lithography where computational efficiency is critical. Here, we introduce a novel modal-superposition method for PCB synthesis, termed “few-mode superposition” and demonstrate its effectiveness in achieving PCBs with higher precision and efficiency. The method requires significantly fewer modes compared to conventional techniques while maintaining high accuracy in intensity and coherence. We apply the few-mode superposition method to the efficient generation of partially coherent light sources and computational lithography, showcasing its ability to rapidly produce PCBs with nonconventional cross-spectral density functions. This facilitates fast lithography simulations and other applications involving partially coherent light. Our approach significantly accelerates both the generation and calculation of PCBs and holds promise for integration with on-chip laser sources, as well as for high-energy laser generation and lithographic mask design.