Abstract: Many experiments have shown that zenith integrated precipitable water vapor(PWV) can be readily obtained with better than 2 mm absolute accuracy using GPS instruments.However,PWV measurements do not provide any information on the spatial distribution of water vapor.The measurements of slant precipitable water vapor(SWV) along each path contain much information which has the potential to be useful for numerical weather prediction.In order to get precise SWV,firstly the principle of remote sensing of SWV and computed diagram are described;secondly improved algorithm of converting double-differenced residuals to zero-differenced residuals is developed;finally a real example is given to demonstrate the algorithm involved using GPS and WVR data at one site.It is shown that the improved algorithm can be used to estimate SWV and the root-mean square error is less than 4mm between GPS and SWV.The accurately estimated SWV can be used to retrieve the three dimensional information of water vapor which is useful for numerical weather prediction,geodetic surveying and interferometer synthetic aperture radar(InSAR).