Abstract: To determine an orthometric height using GPS,it is necessary to know the geoid/ quasigeoid undulation.Many methods have been studied for the interpolation of discrete GPS/leveling data to get the geoid/quasigeoid undulation unknown.A new half free-positioned point masses model is proposed based on equivalent sources theory.Corresponding iterative algorithm to construct the point masses is also presented considering the relationship of the nearest GPS/leveling points.The results in Hong Kong and Shenzhen show that the point masses model is efficient for the interpolation of geoid/quasigeoid undulation.