To eliminate depolarization loss in the side-pumped Nd∶YAG ring resonator single-longitudinal-mode laser,the laser beam reflected by the thin-film-polarizer was used as output of the ring resonator.The experiments show that the ring resonator unidirectional operate and deliver single longitudinal-mode laser beam.There is not depolarization loss in this ring resonator.Under a drive currents of 21A,5.1W 1064 nm single-longitudinal-mode laser beam with a line-width of about 200 MHz was obtained from this ring resonator.The polarization state from the ring resonator is line-polarized light.The output power and the efficiency of the ring resonator can be further increased by use of the end-pumped geometry and the thin-film polarizer with optimum-reflectivity for p-polarized light in the ring resonator.