As BeiDou Satellite Navigation System-3 (BDS-3) starts to provide service for global users, it is possible to get global-coverage and all-time positioning service for space application using BDS alone. The performance of space-borne BDS positioning is thoroughly analyzed with the in-orbit data of GNSS Occultation Sounder (GNOS) aboard FengYun-3D (FY-3D) satellite. Firstly, the visibility and position dilution of precision (PDOP) of BDS satellites in different LEO orbits are calculated based on real BDS ephemeris, and the orbit and clock error of broadcast ephemeris and Signal-In-Space Range Error (SISRE) are studied. The results show that the global coverage usability from ground to 2000 Km height orbit has already been 100%. The mean visible BDS satellite number across the world is 50% larger than that of GPS. For BDS broadcast ephemeris, the 3-D orbit error is 1.5m and clock error is 2.4ns. SISRE is about 0.79m and the clock accuracy of BDS-3 has reached the same level of GPS. Second, the real visible satellite number, signal strength, precision of pseudo-range and position accuracy are verified with the measurement data of GNOS. The code biases of BDS-2 satellites are focused on. The in-orbit data results show that GNOS in FY-3D could get 100% positioning with BDS-2 signals in service areas, and 3-D position accuracy is 5.53m. All BDS-2 satellites including Geosynchronous Earth Orbit (GEO), Inclined Geosynchronous Orbit and Medium Earth Orbit satellites have code biases. When the elevation is less than 40 degree, the code bias of GEO is firstly measured directly. The total electron content above 836Km LEO orbit is measured using BDS dual-frequency measurements, which can cause relative pseudo-range delay of about 0.6m. The research in this paper is of great significance to the space-based application of BDS and lays the foundation for the design of space-based BDS receivers.