The absorption enhancements of silicon layer in silicon solar cells with three kinds of Ag nanoparticles including sphere, cylinder and cuboid are studied by the finite difference time domain (FDTD) method, respectively. The results show that the light absorption of silicon is significantly improved due to the localized surface plasmon (LSP) resonance. The relations of the absorption enhancement with the parameters of nanoparticles are thoroughly analyzed. The optimal absorption enhancement can be achieved by adjusting the relevant parameters. Among the three types of Ag nanoparticles, i.e., sphere, cylinder and cuboid, the silicon with the cubical Ag nanopaticles shows the most efficient absorption enhancement at optimal conditions, its maximum absorption enhancement factor is 1.35, and that with the spherical Ag nanopaticles gets the lowest absorption enhancement. The work is useful for the further theoretical study and design for the plasmonic thin-film solar cell.