Abstract: The difference between GPS leveling and gravity geoid is causedlies in by not only the different datums that they refer to respectively,but also the errors inevitably contained in the gravity and GPS leveling observations.The relationship between the transformation parameters of datums and the difference of these two observation technologies is established,and the parameters and residuals are calculated based on the least-square criterion.When GPS leveling data is more accuracy than gravity data,the geoid can be determined easily by this joint solution model,only considering the errors of gravity observations.