Measured interference fringe period error has a major effect on the phase stitching error in scanning beam interference lithography system. In order to make high quality grating masks which can meet the requirement of the ion beam etching, mathematical model of the relationship between measured fringe period error and dose contrast is established. Based on the photoresist nonlinearities in developing process, the developing model of grating masks made by scanning beam interference lithography system is built. According to this model, variation of groove profile with measured fringe period error is given. Experiments are done to verify this model. It shows that: measured fringe period error makes groove profile worse and varies in space domain. With a certain measured fringe period error, phase stitching error between scans is proportional to step over distance and inversely proportional to fringe period. When interference beam waist radius is 1 mm, step over distance is 0.8 mm and grating line density is 1800 gr/mm, with certain developing conditions, measured fringe period error must be less than 139 ppm to make grating masks with bottom clean without residue and groove uniform.