Ultraviolet laser operating at 355 nm has been found to have wide applications in scientific and industrial fields of laser radar, biological fluorescence medicine, micro processing, laser marking and laser ablation, owing to its superior properties of short wavelength, high single-photon energy, and high resolution. In addition, 355 nm laser plays a vital role in promoting the development of RGB full color display because it can be used as an excitation source for investigating the blue light emitting materials. LiB3O5 (LBO) crystal possesses relatively high nonlinear coefficient and high optical damage threshold. Therefore, it is generally employed to generate 355 nm light through the third harmonic generation (THG) of the Nd:YAG laser (1064 nm). However, the CsB3O5(CBO) crystal, which also belongs to B3O7 group has attracted more attention for its larger nonlinear coefficient. The temperature sensitivity is another important characteristic of the nonlinear crystal. Temperature fluctuation can cause the variation of refractive index of nonlinear optical crystal, which leads to phase mismatch and thus affects the nonlinear conversion efficiency. The principal refractive index of CBO crystal was accurately measured using the auto-collimation method in a temperature range from 40 to 190 ℃ for the first time by Zhang et al. in 2013 [Zhang G C, et al. 2013 Opt. Lett. 38 1594], while the temperature bandwidth of CBO for 355 nm THG has not been reported. In the present paper, a high-power 355 nm laser is produced by efficient THG of an acousto-optic Q-switched quasicontinuous wave 1064 nm laser in CBO crystal. The master-oscillation power-amplification (MOPA) system with Nd:YAG crystal which is side pumped by high-power pulsed laser diode (LD) array delivers 210 W of a quasi-continuous Q-switched 1064 nm laser power. The laser operates at a 1 kHz repetition rate, and each pulse train contains five Q-switched pulses each with a duration of 40 ns. The 98 W of 532 nm green light is produced by second-harmonic generated in type-I LBO crystal. The 28.3 W ultraviolet laser is achieved by a 30-mm type-II CBO crystal through the sum frequency of 1064 nm and 532 nm light. The conversion efficiency from the fundamental light to the third harmonic reaches 13.5%, which is 28.6% higher than that obtained with a type-II LBO crystal under the same experimental conditions. The temperature sensitivity of CBO crystal in the 355 nm THG process is studied. Its temperature bandwidth is 25, which is much broader than that of LBO crystal. The experimental results show that the CBO crystal is superior to LBO crystal in the sense of conversion efficiency and temperature sensitivity for THG of 355 nm.