We develop an AlGaInAs periodic quantum-well (QW) device with a large modulation strength to severs as a saturable absorber in a QCW diode-pumped Nd:YVO4 laser. The saturation transmission of the AlGaInAs QW saturable absorber was measured with a nanosecond 1064-nm Q-switched laser as the excitation source. Measured result indicates that AlGaInAs QW device has a much larger absorption cross section than that of Cr4+:YAG crystal to be an appropriate absorber for passively Q-switching Nd:YVO4 lasers. Stable Q-switched pulses with a pulse energy of 3.5 mJ and a peak power of >1 MW were generated at a pump energy of 34 mJ. The fluctuation of the output pulse energy at a repetition rate of 200 Hz was found to be less than ±2%.