We employ first-principles to calculate the adsorption energy, the band structure, the density of states, the charge populations, the work function and the optical properties of 1/4ML Cs adsorption on (2 2) GaN(0001) surface using the density-functional theory within a plane-wave ultrasoft pseudopotential scheme. The results show that the most stable position of Cs adatom on GaN(0001) surface is at the bridge site of N atoms for 1/4 coverage. The surface of GaN(0001) shows still metallic character after adsorption. Cs adatom affects mainly Ga atoms at surface. The transfer of Cs6s state electrons to Ga atoms at outmost layer leads to the decrease of work function. By analysis of optical properties, we can see imaginary part of dielectric function, absorption spectrum and reflected spectrum shift toward low energy after Cs adsorption.