Position accuracy is one of the most important parameters of GPS. Differential GPS(DGPS) can get higher position accuracy significantly, but there are many limitations for its applying ranges, so improving the accuracy by independent positioning method has important significance. Experiments in the non-differential mode by using Trimble AgGPS132 were conducted. First, the frequency distribution of the experimental data was analyzed. A method to eliminate outliers was raised, and GPS precision operating was compared before and after eliminating outliers. Finally, how to eliminate the influence of error was pointed out according to the different conditions. The results showed that adopting the Levin special criteria to eliminate outliers could improve the positioning accuracy effectively, and adding control point could correct the drift data of GPS dynamic positioning.