A partial field optical system baffle was designed and optimized. Partial field was non-symmetry along one direction, so a large number light tracing was needed to design the centric, which made it difficult to design. A method of footprint diagram was used to get the coordinate of special light in the space, the size of effective field and the part of baffle blocking light. The part blocking light was removed and a "duck mouth shaped" baffle was got, which can decrease the difficulty of design. By optimizing the design, length of final designed baffle was half of the former one, and reduced the size along Y direction, which can reduce more stray light to enter the later optical system directly. Baffle was modeled and simulated by Solidworks and TracePro software, the PST is about 10-9 when the off-axis angle is 30°, which is less than 5.59×10-7, the requirement of the system. The result of the simulation shows that using method of footprint diagram to design inner baffle of a partial field optical system is feasible.