A single-frequency passively Q-switched laser was constructed, in which a co-doped crystal served as an active element, a mode selector, and a passive Q-switch simultaneously. In order to obtain the frequency instability of 10^(-6), a stable single-frequency operation was presented and its characteristics were determined. The experimental results showed that the stable single frequency could be maintained for half an hour and the linewidth was approximately 530 MHz at a pump power of 13 W.