A laser diode with the central wavelength of 888 nm is used as the pump source to mitigate the thermal load in the pumping progress. Through the optimization of the ring cavity, the mode size in the Nd:YVO4 crystal is enlarged, and the astigmatism of the cavity is minished. A figure "8" shaped ring cavity is employed to enforce the single-frequency operation. As a result, a stable, high-power, single-frequency green laser (M2<1.1) of 21.5 W is obtained with the optical-optical conversion efficiency of 31.9%, and power stability of ±1% for 8 h.