By determining the relations between the effective distribution coefficient of MgO in the congruent LiNbO3 and the phase-matching temperature and the concentration of MgO, chemical composition which makes the Mg :LiNbO3 to get the highest phase-matching temperature has been found. The problems frequently encountered in heavily doped Mg:LiNbO3 crystal growth, such as growth striations and precipitation etc, have been resolved.The Mg:LiNbO3 crystals with high resistance to light-induced refractive index changes and good optical quality have been grown. Using the crystals for frequency doubling of a continuously pumped acousto-optical Q-switch Nd :YAG laser, second harmonic output with the highest avarage power of 2W have been obtained.