An all-fiber Q-switched Yb3+-doped fiber ring laser pumped with a 976 nm laser diode (LD), which uses a fiber-pigtailed acousto-optic modulator (AOM) as the active Q-switching element, was reported. The stable pulse can be obtained when the modulation frequency was adjusted from 200 Hz to 60.9 kHz. The laser pulse with maximum peak power of 2.7 W, pulse width of 53.2 ns and pulse energy of 145.5 nJ is obtained, when the pumping power is 183 mW and the modulation frequency is 500.2 Hz. The alternate ouptut of high and low energy pulse is observed with the shorter Q-switched turnoff time, and it is explained by the principle of the Q-switched laser. The characteristics of pulse versus modulation frequency and pump power are investigated, and the theoretical calculation is in good agreement with the experimental result.