An all-solid-state single-longitudinal-mode passively Q-switched laser in a ring cavity is demonstrated. The laser diode (LD) side-pumped ring cavity is adopted to eliminate the spatial hole burning effect to stabilize the number of longitudinal modes in the resonator, and the etalon is used to control the net gain difference between adjacent longitudinal modes of the laser to realize the operation of a high single-longitudinal-mode ratio laser. The laser operates at a repetition rate of 10 Hz, producing pulses with a pulse width of 23.6 ns and a pulse energy of 6.1 mJ. The laser has strong stability in output energy and single-longitudinal-mode ratio, a relative standard deviation of 1.56% is demonstrated, and a single-longitudinal-mode ratio of 100% is achieved when 10000 pulses are recorded continuously.