A LD side-pumped Nd∶YAG all-solid-state ultraviolet laser is demonstrated under acousto-optic Q-switched operation, by using a simple and compact three-mirror folded and flat-concave cavity resonator, with type Ⅰphase-matched LiB3O5 (LBO) as second harmonic generation crystal and type Ⅱ phase-matched LBO as third harmonic generation crystal. The output power of 355-nm UV laser reaches to 2.14 W at the incident pump power of 155 W and repetition rate of 5.40 kHz with pulse width of 45 ns. The light-light conversion efficiency is about 1.38 %, and the power stability is 3.30% in 1 h operation accordingly. The influence factor on the triple frequency conversion efficiency is studied.