The use of the surface plasmon ploritons(SPPs) instead of photons as the exposure source can pattern nanoscale feature size by its near field enhancement effects. In this paper, we numerically explore the parameter spaces in prismbased SPP lithography system. The calculation principles and methods are given out. Results show that high refractive indices of prism, low thickness of silver film, appropriate incidence wavelength and the refractive indices of resist contribute to optimal interference image with high expore depth and contrast. When chosing 40 nm silver film at 431 nm incidence wavelength, exposure depth achieves 200 nm, and fringes period is 110 mm. The numerical results offer theoretical support for the arrangement of the experimental setup.