The careful theoretical and experimental study was performed on the relationship between laser output power and beam quality for the cw laser diode pumped Nd:YAG rod laser. In the condition of LD side pumping, the pumping uniform and the absorption efficiency to pumping light was compensated by optimizing the laser resonator with dual rod structure. The thermal induced birefringence was compensated by inserting polarization rotator and optical image system between two rods. The average laser output power of 300W with 25% light-light conversion efficiency was obtained when the total pumping power was 1200 W. The Q-switched laser beam at repetition rate of 15 kHz with average output power of 240 W, beam quality Af of 18 and pulse width of 110 ns was obtained too.