A Q-switched Nd∶YAG oscillator for seed injection was designed to develop all solid state single-frequency Q-switched laser.The cavity length of the oscillator and the optimum transmittance of the output coupler are identified via theoretical analysis together with index requirement.In the experiment,the oscillator has maximum output power of 8.21W when operating at free-running mode.The optical-to-optical efficiency is 38% and the slop efficiency is 47.7%.A pulse power of 4.28 W and a pulse width of 6.6 ns are obtained in the Q-switched operation,corresponding to the slop efficiency of 28.9%.The pulse energy is 8.6 mJ and the pulse peak power is 1.3 MW.The output beam qualities of Mx2=1.22(horizontal direction) and My2=1.29(vertical direction) are tested,which is fraction-limited.The results of the experiment is satisfied to the anticipated index requirements.