Relations between light wavelength and phase-match angle, efficient nonlinear factor, walk-off angle are theoretically analyzed by using coupling wave equations for third harmonic wave generation (THG) in BBO crystal. THG experiment is done by using all solid-state Q-switching solid-state laser and phase-match of type 1 for BBO crystal. Ultraviolet light pulse energy of 1.5 mJ/pulse at 355 nm is achieved.