A nanosecond optical parametric oscillator (OPO) pumped by an electro-optic Q-switched Nd∶YAG laser with 532 nm green light (10～13 ns, 0～20 mJ) was constructed. The resonant cavity of 40 mm long was made from a BiB3O6 crystal of size 4 mm×4 mm×12 mm with type-Ⅱ phase matching (θ=57.5°, Φ=0°), and a pair of plane mirrors with singly-resonant wavelength 1215 nm. The OPO system converted the incident pump beam into the signal laser of 1215 nm and the idler laser of 946 nm. The pump threshold of 9 mJ and the highest coupling output of 2.57 mJ for the highest conversion efficiency of 14.59% were achieved.