A multi-GNSS precise point positioning (PPP) algorithm considering the inter-system bias (ISB) is adopted to process the multi-GNSS data obtained from 7 stations of the multi-GNSS experiment (MGEX). Using the proposed algorithm, the ISBs values can be achieved between Galileo, GLONASS, BDS (BeiDou Navigation Satellite System) and GPS (Global Positioning System). The results of static multi-GNSS PPP solutions show that RMS (root mean squares) values are 8.9 mm, 5.3 mm and 10.9 mm for the east, north and up directions respectively. One-day stabilities of ISBs described by STD (standard deviation) values are better than 0.12 ns for different systems, and especially Galileo is the best one. From the sequence of multi-day ISBs, significantly irregular ISB jumps can be found where the change range can reach nearly 20 ns. There are some differences on ISB values for different types of receivers, and the ISB values are similar for same types of receivers. On the whole, the ISB for Galileo is the most stable and optimal, and the results of BDS and GLONASS are almost equivalent.