Abstract: Due to the inconsistency of geodetic-height datum between two adjacent GPS networks,there exists a height-anomaly difference in same GPS point lied the common area of the two networks.The difference results in unable building a quasi-geoid in large-region with these historic GPS-net data.In order to solve the problem,two methods are studied in detail.One is unifying vertical datum;the other is building height-anomaly difference model.For the first,according to the theory of 3-dimension unconstrained adjustment,the two height anomalies of a GPS/leveling point in adjacent GPS networks and the geoid models established respectively in different GPS networks,a geoid model in large-region is constructed finally.For the second,using the way to unify vertical datum in adjacent GPS network,integrating the stability of height-anomaly difference between two points,firstly the height-anomaly difference of each point relevant to the height anomaly of a given GPS/leveling point is calculated.Then,build a model of height-anomaly difference with a polynomial function is built.In virtue of the polynomial function and the given GPS/leveling point,the height anomaly of any GPS point in the GPS network can be calculated.The two methods are tested with 3 historic GPS-net data in north shore of Changjiang estuary,and ±4 cm of root of mean square is acquired.