An injection-seeded, doubly resonant ring cavity optical parametric oscillator based on PPMgLN crystal is presented. The pump beam polarization matches the e→e+e interaction in PPMgLN crystal (doped MgO with mole fraction of 5%), and thus makes the maximal nonlinear coefficient d33 (25 pm·V-1) effective. Using single-frequency pulsed laser with repetition frequency of 400 Hz , wavelength of 1064 nm and output power of 556 mW as pumping source, we obtain an average output power of 79 mW at the wavelength of 1.57 μm and output power of 38.5 mW at the wavelength of 3.3 μm when the crystal grating period is 30.5 μm and temperature is 110 ℃, corresponding to a total conversion efficiency of 22%. By using the seed laser injection technology, we obtain single-frequency narrow linewidth laser output, the linewidth of the signal laser with wavelength of 1.57 μm is less than 100 MHz, and the frequency drift is less than 141 MHz in 10 minutes.