The differences of the maximum output powers and optical-to-optical conversion efficiencies of the intracavity doubling single-frequency Nd:YVO4 lasers pumped by 808 nm and 888 nm laser diodes are compared. The results prove that direct pumping is an efficient method to improve the intracavity doubling laser performance. However, due to the contradiction between the absorbed efficiency and nonradiative transition under 888 nm pumping, the doped concentration of Nd:YVO4 crystal is an important factor for the laser design. By analyzing the influence of Nd3+ doped concentration on the lasing performance in experiment and theory, a Nd:YVO4 crystal with doped concentration of 0.8% (atomic fraction) is chosen as gain medium. At last, a high-power single-frequency 532 nm laser of 21.5 W is obtained from Nd:YVO4 crystal with Nd3+ doped concentration of 0.8% pumped by 888 nm semiconductor laser, and the corresponding optical-to-optical conversion efficiency is 31.6%.