Currently, analysis centers(ACs) of International GNSS continuous monitoring and assessment system (iGMAS) could provide precise GNSS orbit products. To improve the stability and reliability of orbit products for high-precision users, we usually combine the orbit products from ACs. In this paper, we present a robust least square method of combing satellite orbits from different solutions. We first verify this method by combining GPS and GLONASS final orbit products provided by IGS Analysis Centers. Results show that the RMS differences between our combined GPS orbits and those from IGS are about 4 mm, and the RMS differences between our combined GLONASS orbits and those from IGS are about 5 mm. As the orbits of different IGS ACs are calculated in different reference frames, we first transform the orbits of different IGS ACs to the reference frame of IGS combined solution independent exchange format (SINEX) solutions before orbit combination. Results show that this transformation can improve the quality of combined orbits up to 2 mm. We then apply this method to analyze and combine GNSS satellite orbit products provided by ten of iGMAS ACs. For most iGMAS ACs, results show that the precision of GPS final and rapid orbits are better than 2.5 cm, and the precision of GPS ultra-rapid orbits are better than 6 cm and 15 cm for observed part and predicted part, respectively. The precision of iGMAS combined final orbits are 2 cm, 2~3 cm and 6 cm for GPS, GLONASS and Galileo satellites, respectively. The precision of iGMAS combined final orbits are 1.5 m and 20 cm for IGSO and MEO/IGSO of BDS, respectively. The relatively lower precision for combined GEO orbits is due to the different satellite antenna phase center offset (PCO) models adopted by different iGMAS ACs.