The study proposes a geographic information systems (GIS)-based slope stability analysis method assuming a normal stress distribution acting on the slip surface. Compared with traditional methods, the three-dimensional (3D) safety factor acquired through this method will more closely approximate the actual value. First, a 3D slope stability analysis model is developed using grid column units, and the spatial expression of calculation parameters based on the grid column is given by the spatial analysis capability of GIS. Then, four equilibrium equations are derived under the limit equilibrium condition. The normal stress distribution acting on the slip surface is analyzed to construct a reasonable normal stress distribution approximation function. The 3D safety factor is obtained through the approximation function and the Mohr-Coulomb strength criterion. Moreover, we develop a GIS-based extension module which combines the grid-based data with the 3D slope stability analysis model. The accuracy and feasibility of the module are verified by three typical cases.