Abstract: A lamp pumped high peak power electro-optical Q-switched Nd:YAG 266 nm UV laser was demonstrated. In combination with the KDP crystal properties, and based on the frequency doubling theory, the influence of KDP crystal length on the conversion efficiency under the condition of existence phase mismatching was analyzed considering the walk-off effect. The compact flat cavity structure was used and the 1064 nm laser generated from lamp pumped electro-optical Q-switched Nd:YAG laser was used as fundamental frequency laser. A type Ⅱ phase-matched KTP crystal and a type Ⅰ phase-matched KDP crystal were used for the extra-cavity second-harmonic generation and fourth-harmonic generation respectively. Using energy meter, oscilloscope and other instruments, the UV laser pulse energy of 35 mJ/pulse at the repetition rate of 1 Hz and the pulse width of 6.0 ns at 266 nm were achieved, the peak power was 5.83 MW. UV laser pulse energy of 28.9 mJ/pulse at 266 nm was achieved at the repetition rate of 10 Hz. A conversion efficiency of 532 nm to 266 nm was up to 31.9%. Laser marking and laser engraving can be achieved with this high peak power, short pulse width 266 nm UV laser.