A new method of optical pre-pumping was proposed for active Q-switched lasers. With the optical pre-pumping the stability of Q-switched lasers would be improved effectively by squeezing intermediate frequency noise. Experiments showed that the stability of Q-switched output pulses would be increased by 4% in the system of LD pumped acoustic-optical Q-switched Nd:YAG. At the repetition of 50 Hz, 45 μJ output energy with single fundamental mode and ±0.8% instability were achieved.