Abstract: The effects of inter system bias (ISB) and inter frequency bias (IFB) need to be taken into account in the multi-GNSS precise orbit determination. In this paper, the ISB/IFB estimation model in the context of the multi-GNSS orbit determination is derived. A constraint method for reduce the rank defect about ISB/IFB is presented based on the GPS datum. Experimental results from MGEX data indicate that ISBs/IFBs of the different system are of good stability, and have high consistency for satellite time series in each system. The standard deviation of the BDS ISB, Galileo ISB, GLONASS IFB are 0.36 ns, 0.18 ns, 0.51 ns, respectively. The ISBs from different monitoring stations are relatively closed to those stations with the same type receiver, but ns-level differences can be still achieved. The GLONASS IFBs are consistent with the same channel number satellites of the same monitoring station and different monitoring stations.