The positioning accuracy requirement of mobile measurement system is expounded. The three methods of positioning accuracy evaluation including the circular probability method, mean square error method and median error method are analyzed and compared. The positioning accuracy test of vehicle-mounted GNSS/INS is carried out in the standard test field. Experiments show that the evaluation result of the median error method is the most accurate, and its error is less than 1 m, which is obviously better than the circular probability method and the mean square deviation method.