Orbit error is a very important error source of water vapor sounding used in GPS.In order to deal with the orbit interpolation in realtime water vapor sounding.The orbit errors effect on water vapor sounding is analysed and the feasibility of precisely predicting ephemeris using Neville algorithm is studied.It is shown that the Neville algorithm can meet precisely predicted ephemeris interpolation on near real-time and lay a foundation for water vapor sounding.