Ultraviolet (UV) 266-nm nanosecond (ns) laser was obtained through fourth-harmonic-generation by using new nonlinear optical crystals of NaSr₃Be₃B₃O₉F₄ (NSBBF) with different lengths. Since NSBBF crystals are prone to crack during laser experiments, we added a water-cooling device under NSBBF crystal in the experiment, and compared the experimental results under conditions of water cooling and no water cooling. Moreover, different laser focusing effects were explored to optimize the experimental conditions. The maximum optical conversion efficiency from 532 to 266 nm was 4.8% with the pump power of 2.8 W. The maximum average output power of the 266-nm UV laser was 420 mW when the 532-nm pump power was 23.82 W. The study provides research ideas and experimental basis for NSBBF crystal to realize higher-power 266-nm ns laser output.