A high-average-power and high-beam-quality diode-side-pumped solid-state Q-switched laser was developed by use of low concentration Nd:YAG crystals with thermally near-unstable resonator design and two-rod birefringence compensation technology. Two orthogonal acousto-optic Q-switches were used to sustain fully holding of the pump gain. The 1064 nm quasi-continuous-wave average output power of 258 W at pump power of 1116 W was achieved with the beam quality factor M2～15.5 at a repetition rate of 10 kHz and a pulse width of 64 ns, corresponding to a peek power of 0.4GW and an optical-to-optical conversion efficiency of 23.1%.