An acousto-optic Q-switched Nd:YVO4 laser end-pumped by laser diode (LD) is used as the pump source, and a MgO-doped PPLN (PPMgLN) is chosen as nonlinear crystal. By optimizing the pump beam quality and mode matching, a mid-infrared pulsed optical parametric oscillator (OPO) is realized. In the experiment, if laser diode (LD) pump power is determined, mid-infrared laser output power is changed by the repetition frequency of the acousto-optic switch. When the repetition rate is small, the output power increases as the repetition rate increases. But when the repetition rate reaches a certain value, output power decreases as the repetition frequency increases. The optical parametric oscillator could be tuned from 2.95~4.16 mm by changing the period of PPMgLN.