Abstract: In recent three decades, the development of Global Positioning System (GPS) highly extends its application fields, one of which is to use GPS monitor water vapor in the troposphere. When Slant wet delays of GPS signals are treated as tomography observations, they can be used to retrieve three-dimensional wet refractivity fields of the troposphere. However, due to the uneven distribution of the signal rays and flat orography of the network in Hong Kong, the tomography observation equations are ill-posed, so some constraints are usually added to determine the unique tomography solution. since the water vapor changes rapidly in the vertical direction, appropriate vertical constraints play an important role in retrieving the accurate vertical structure of the wet refractivity fields. By investigating the vertical distribution characteristics of the wet refractivity in the atmosphere in Hong Kong area, we find that the Gaussian function could well express the relationship between the wet refractivity and the height. The tomographic experiments using data from Hong Kong Satellite Positioning Reference Station Network shows that tomographic solution using Gaussian function to establish vertical constraints gets a better agreement with the radiosonde data and the European Centre for Medium-Range Weather Forecasts (ECMWF) data when compared with the solution obtained by establishing vertical constraints by using the exponential function. The improvements with respect to standard deviation are 3.8 mm/km in the whole troposphere and 4.7 mm/km in the lower troposphere. The experiments also show that using good a priori wet refractivity from other meteorological data sources, like radiosonde, could help obtain good tomographic solutions.