The performance of a single-frequency Ho:YAG laser with bow-tie cavity injected by different polarized seed lasers is studied both theoretically and experimentally. With vertically polarized (s-polarized) seeding, a 9.7 mJ single-frequency laser with a pulse width of 183 ns at the repetition rate of 252 Hz is obtained. With horizontally polarized (p-polarized) seeding, an 8.8 mJ single-frequency laser with a pulse width of 220 ns at the repetition rate of 252 Hz is obtained. The laser operating in p-polarized seeding mode performs better than the one operating in s-polarized seeding because of the stable relative center frequency of the pulses (0.9 MHz in 1 h).