A small-size, high-efficiency, liquid-cooled, electro-optic Q-switched laser is obtained, by using the expanded beam lens group with rational designed resonator parameters. The electro-optical conversion efficiency of laser is 1.4% and the output energy of single pulse is greater than 100 mJ. The cavity length is 250 mm with the laser pulse width of 10～15 ns. The beam divergence angle is 1.6 mrad when the laser works on 5 Hz, while 1.8 mrad working on 20 Hz. The output laser beam diameter is 5 mm and the energy fluctuation is less than 5%.