This paper reports the experimental results on the frequency doubling characteristics of Mg:LiNbO3 crystals (phase matching, photorefractive effect), and the parametric optimization for intracavity frequency doubling. Using Mg:LiNbO3 for intracavity frequency doubling of an acousto-optic Q-switched (3KHz) Nd:YAG laser (3.6 kW, pumping power), output of green light with an average power of 2W, pulse width of 300 ns, peak power of 2.3 kW and conversion efficiency of 34% at 0.53μm have been obtained.