An 1.85 W laser diode (LD)-side-pumped Nd:YAG extra-cavity fourth harmonic generation (FHG) all-solid-state 266 nm ultraviolet laser is reported. The high-beam-quality high-stability linear-polarized 1064 nm fundamental frequency laser is obtained from thermal-induced birefringence-compensated resonator with two Nd:YAG rods. The type-Ⅰnoncritical phase-matched LBO and type-Ⅰphase-matched BBO crystals are used as the extra-cavity frequency doubled and FHG crystal respectively. 1.85 W ultraviolet laser of 266 nm is obtained at the pulse repetition rate of 10 kHz with the conversion efficiency of 13.3% from green to ultraviolet light. The self-heating effect of BBO crystal on FHG phase-matched angle is studied, and the result that the self-heating effect can be reduced by adjusting the angle of BBO is proposed.