In a scanning beam interference lithography (SBIL) system, the exposure spot size has significant influence on the stitching accuracy of interference fringes, efficiency of grating fabrication and quality of interference field. In order to obtain a reasonable exposure spot size, the influence of exposure spot size on nonlinear error of interference fringe, line stitching error and exposure contrast is discussed by numerical simulation based on the transmission law of Gaussian beam and scanning stitching mathematical model. Results show that, compared with the large-sized spot, the small exposure spot is more conducive to controlling nonlinear error of interference fringe. Because there exists periodic measurement error, a small exposure spot helps to reduce the line error after stitching and enhance the exposure contrast. The exposure optical path of SBIL system is designed and optimized. The left and right spot morphologies of interference field and the nonlinear error of interference fringe phase are measured. Results show that the waist radius of exposure spot is about 0.9 mm, and the nonlinear error peak-valley value of interference fringe phase is 21.8 nm.