The adsorption mechanism of water on the GaN (0001) polar surface is investigated via both the Density Functional Theory (DFT) method and its derived classical force field. The physisorption binding energy and the adsorption geometry of the water molecule on the clean Ga-terminated surface are analyzed via the first-principle static calculations. The adsorption energy hypersurfaces are then extracted to be used in the fitting of the interaction potentials between water and GaN. Classical molecular dynamics (MD) simulations based on the developed force field are performed for the interfacial system of liquid water and the GaN surface slab. From our computations, the interfacial water exhibits significant oscillatory profiles for the atomic densities and the molecular orientations. Further data analysis suggests a highly confined first layer with the O being locked right upon the surface Ga atoms and the H pointing toward the neighboring O to form the weakened hydrogen bonds. A bilayer configuration with opposite dipole orientations is consequently characterized as the wetting structure on the GaN polar surface and is explained by the anisotropic perturbations from the surface polar sites. Our simulations would be helpful to provide an atomistic picture for the water adsorption configuration on this semiconductor surface and would be useful in the relevant nanofluidic and nanoengineering applications.