Abstract: The satellite Fengyun 3C(FY3C) launched in 2013 and carried a global navigation satellite system(GNSS) occultation sounder, which is the first BDS/GPS compatible radio occultation sounder in the world and records both GPS and BDS measurements. It provides an opportunity for investigating the precision real-time orbit determination based on space-borne GPS/BDS measurements. In order to realize the real-time orbit determination based on space-borne GPS/BDS measurements with decimeter-level accuracy, this paper carries out in-depth research in the following aspects. Firstly, the real-time orbit determination mathematical model based on the space-borne GPS/BDS carrier phase observations is introduced. Then the properties of the errors in the signal's propagation path that originates from GPS/BDS broadcast ephemeris are analyzed, and the signal-in-propagation path error(SIPPE) statistics of different kinds of GPS/BDS satellites are also presented. Based on the analysis, a pseudo-ambiguity is modeled as a stochastic process and estimated in the real-time orbit determmination(RTOD) filter to absorb the SIPPE error, and then the decimeter-level accuracy can be realized. The space-borne GPS/BDS data from China's FY3C satellite are employed for the simulative RTOD test using the SATODS software. Results have demonstrated that the position and velocity accuracy of 30 cm and 0.3 mm/s, respectively, are achieved when using GPS/BDS carrier-phase data and broadcast ephemeris. Although the accuracy of BDS broadcast ephemeris is obviously worse than that of GPS, orbit results of the GPS/BDS fusion solution with reasonable stochastic models for pseudo-ambiguities will be slightly superior to that using single GPS data. For example, when the FY3C satellite flew over Asia/Pacific region where more BDS satellites can be tracked, the orbit accuracy can be improved by 10% with the GPS/BDS fusion solution.