The inner light baffle is introduced due to the layout of coaxial two reflect mirror system for stray light elimination. Different point source transmittance (PST) curves of the system are gained by changing diameter and height of the inner photomask which is quite useful for optimization of the inner photomask. By stray light analysis and finite element analysis (FEA), it′s ascertained the diameter of the inner photomask is 44.8 mm and the height is 100 mm, then great suppression effectiveness for infrared stray radiation is achieved together with required mechanical capability for the coaxial two reflect mirror system with an aperture of 320 nm.