Satellite Based Augmentation System (SBAS) provides user differential range error (UDRE) to ensure accuracy of broadcast ephemeris and satellite clock corrections. The paper presents an improve algorithm for UDRE calculation by creating status and measurement equations of satellite navigation system clock as well as ephemeris error, calculating correction data with accuracy estimation matrix of ephemeris and clock error by Kalman filter, then calculating UDRE based on filter error estimation accuracy. Simulation on the impact of reference station on satellite UDRE is carried out through analyzing the effects of reference stations’ mask angles and numbers on the TDOP and UDRE of three different types of satellite. Simulation analysis proves that the mask angle of reference stations is 15 degree, and the umber of reference stations is more than 20.