A diode-end-pumped Nd:YAG intracavity frequency-tripled quasi-continuous 355 nm laser has been demonstrated. The efficient third-harmonic generation with high peak power and stable operation is generated by a lithium triborate （LBO） crystal successfully. The laser cavity is a simple plane-to-plane linear cavity, which is short and compact. An average 355 nm laser output power of 163 mW is obtained at a pump power of 5.73 W and a pulse repetition rate of 9 kHz, with the highest optical-to-optical efficiency of 2.84%. The highest average 355 nm laser output power is up to 174 mW at a pump power of 6.7 W and a pulse repetition rate of 5 kHz. The instability of the output power of 174 mW is less than 5% and the beam quality M2 factor is 3.79. An average 355 nm laser output power of 112 mW is achieved at a pump power of 5.73 W and a pulse repetition rate of 2 kHz and the peak power reaches the highest value of 9.15 kW. Compared with the technology of external cavity frequency conversion, the technology of intracavity frequency conversion can cut down some of the optical elements in the laser cavity, reduce the size of the laser, and enhance the handiness of the laser. These advantages will benefit the wide applications of the ultraviolet lasers.