Using digital zenith instrument for positioning the observation station is an astronomical positioning method with high accuracy.In calculating the position of observation station,there lacks an effective method for screening the star data.Direct processing of the star data obtained from experiment always has an influence on the final positioning accuracy.In order to solve the above problems,a method for star screening is proposed.With residual error as the object of study,it uses Grubbs criterion for repeating screening of star data in iterative computation,and then uses the least square algorithm for calculation.The processed result of experimental data has rejected the stars with gross error and improved the reliability of calculated data,which can improve the positioning accuracy by about 5 m.