A 444 nm blue laser diode and a 469 nm blue laser diode, whose maximum pump powers are 1.4 W and 1.5 W, respectively, are used as the pumping source. The two blue laser diodes are fixed in the π polarization direction. A 45° combiner is used to combine the pump light of the two blue laser diodes. An aspherical lens with a focal length of 4 mm is used as a collimator to collimate the pump light emitted by the blue laser diode. A spherical lens with a focal length of 12 mm is used as a focusing lens to focus the pump light emitted by the blue laser diode. The cavity structure of the laser is V-shaped. The Fabry-Perot etalon is used to select a single-longitudinal mode. The Pr∶YLF crystal with a size of 3 mm×3 mm×5 mm and doping concentration (mass fraction) of 0.5% is used as the gain medium. The type-I phase-matched BBO crystal with a size of 3 mm×3 mm×3 mm is used as the frequency-doubling crystal. By optimizing the resonator parameters and the parameters of two Fabry-Perot etalons, the maximum output power of 261 nm single-longitudinal ultraviolet laser is 110 mW when the incident pump power is 2500 mW.