It has been proved that zero-differenced (ZD) ambiguity resolution (AR) can achieve better accuracy and reliability in multi-GNSS data proceeding. However, the current AR for precise orbit determination (POD) is usually based on double-differenced (DD) ambiguity. In this contribution, a ZD AR approach for GECR four-system POD is developed based on a multi-GNSS UPD estimating strategy. Observations from over 150 global distributed MEGX and IGS stations are selected for the estimation of GECR four-system UPDs as well as zero-differenced integer ambiguities, with which the carrier-phase observations can be converted to carrier-range observations. Thus ZD AR for GECR-combined POD can be processed efficiently with the carrier-range observations. Meanwhile the integer recovered clock (IRC) of four systems can also be obtained. For the GECR-combined UPD estimation, the wide-lane (WL) UPD results of GPS and Galileo satellites are stable within one month and the daily changes of narrow-lane (NL) UPD results are smaller than 0.2 cycles, while the results of BDS and GLONASS satellites are slightly worse. As for the GECR-combined POD results based on ZD AR, the day boundary discontinuities are 2.0 cm and 3.9 cm for GPS and Galileo satellites, with improvements of 14.8% and 17.2% compared with DD AR, and improvements of 57.0% and 56.6% compared with float resolution. Moreover, the SLR validation for Galileo satellites also shows that the ZD AR has comparable mean bias but smaller STDs compared with DD AR. Furthermore, the computing time of POD with ZD AR is reduced significantly as well.