The periodically poled characteristics of LiNbO_3 doped 5mol% MgO were investigated and the domain reversal field was found to be 3kV/mm. According to the influence of MgO doped on the crystal defect, the decrease reason of the domain reversal field was explained. By using short-pulse field, the periodically poled grating was successfully fabricated in 1.0mm thick MgO:LiNbO_3. Second harmonic green light generation in periodically poled MgO:LiNbO_3 was demonstrated. The green light power of 3.5mW at 532nm is obtained when the fundamental power is 75mW, and the conversion efficiency is 4.6%.