A simple laser-diode pumped acoustic-optic Q-switched fiber laser is reported by using China-made large-mode-area ytterbium-doped fiber. Q-switched pulses with a beam quality factor of M2 about 2 and several hundred nanoseconds pulse duration are achieved at the repetition rate of 1-50 kHz. When the repetition rate is 1 kHz, the pulse energy is 0.93 mJ with the pulse duration of 132 ns. Meanwhile, the profile of laser pulses shows some mode-locking phenomena, the mechanism of the phenomena is discussed.