Abstract: In general, the positioning accuracy of single point navigation system is only about 15m, It can't satisfy some fields of precision positon, Differential positioning can be a good solution to the problem of low positioning accuracy.In this paper, On the basis of differential GPS, the BDS/GPS combined pseudorange differential system was designed, the user's positioning accuracy can be significantly improved by pseudorange correction generated by the scheme.At the same time, duing to the Beidou system of pseudo range differential message format isn't defined int the RTCM2.X format, this paper proposes an improved RTCM differential message format to make it applicable to the user of the Beidou system, so as to facilitate the Beidou user to use this data transmission protocol for differential positioning