At present, BDS/GPS/GLONASS operates independently. Because of the difference of orbit accuracy, observation noise and other factors, the number of satellites received and the geometric configuration of each system are different. Based on the RINEX format observation data, we can not simply add the fixed weight ratio. Real time dynamic adjustment is needed based on the data quality of the receiver. In this paper, based on the HELMERT variance component estimation model, the optimal weight ratio is calculated by real-time iteration to improve the positioning stability and accuracy. The iteration times of the previous epoch are used as the initial values of the next epoch to reduce the number of iterations. The calculation of the measured data shows that compared with the traditional fixed weight ratio, the positioning accuracy of this method is significantly improved in all directions. Especially under the condition of poor observation conditions, the positioning accuracy and stability are ensured.