A intra-cavity tripe frequency 355 nm ultraviolet laser with high power and high efficiency is reported. Type-II phase matching is used both in second harmonic generation and third harmonic generation. Polarization states of harmonic lasers are selected to improve the conversion efficiency from the fundamental laser at 1064 nm to the third harmonic laser at 355 nm. A simple linear resonance cavity is composed of a laser diode side-pumped Nd∶YAG module, two acousto-optic Q-switches, two LBO crystals separately as a second harmonic generator and a third harmonic generator. A maximum output power at 355 nm of 51.5 W is obtained at repetition rate of 10 kHz under the optical pump power of 623 W. The corresponding pulse width is 36.5 ns at 355 nm, the optical-optical effiency is more than 8% . The measured beam quality factor in x and y-directions are M 2x =10.89, M 2y =12.32, respectively.