Abstract: The number of visible satellites is increasing in the age of multiple global navigation systems. The global navigation satellite system (GNSS), and Chinese BeiDou satellite navigation system (BDS) offer three frequencies signals to users, the ambiguity resolution is improved the higher accuracy and reliability of GNSS-RTK positioning.In general, ionosphere-free combinations are used in single-epoch RTK(Real-Time Kinematic)for medium-baseline. But this algorithm amplifies the measurement noise and the ambiguity resolution performance deteriorates as the length of baseline increases. In this paper, a single-epoch and multi-frequency RTK positioning algorithm for medium-baseline is presented, based on the BDS/GPS dual system combination. Two BDS extra-wide lane ambiguities were fixed at a high success rate and the BDS wide-lane ambiguity was sloved through a simple transformation. Ambiguity-fixed WL was used to improve the success rate of GPS WL ambiguity fixing, and parameterization strategy of ionospheric delay error was used to improve the success rate of BDS/GPS NL ambiguity fixing. This algorithm was validated with real-time data. The results show that the proposed algorithm ia feasible for ambiguity resolution and positioning accuracy.