Abstract: Continuous-wave single-frequency 266 nm laser has a series of advantages such as high photon energy and high processing resolution. It has an irreplaceable role in optoelectronics, precision machining, spectral analysis and other fields, and becomes one of the research focuses in the field of ultraviolet laser at home and abroad. Through theoretical analysis, the annular frequency doubling cavity structure is selected. and the BBO crystal with class Ⅰ angle matching, combined with H-C frequency locking technology is used to carry out the resonant frequency doubling of the external cavity. A continuous 266 nm ultraviolet laser with simple structure and small volume is designed and built under the premise of ensuring high beam quality and high conversion efficiency. When the pump power of 532 nm is 6 W, the maximum output power of 266 nm laser is 1.47 W, the maximum light-to-light conversion efficiency is greater than 24.5%, and the beam quality factors M_X^2 =1.105 and M_Y^2 =1.136 are measured under the output condition of 540 mW. The sum of the final overall structure volume of the scheme is less than 0.05 m³. It can be effectively used in the fields of nanoscale lithography and semiconductor chip wafer inspection.