A NaSr3Be3B3O9F4 nonlinear optical crystal is prepared by a molten salt growth method as follows: uniformly mixing NaSr3Be3B3O9F4 and a fluxing agent in proportion, heating to 850 DEG C, cooling to 2-10 DEG C higher than saturation temperature after reaching constant temperature so as to obtain a high-temperature solution; putting a seed crystal into the high-temperature solution, rotating the seed rod, cooling to the saturation temperature, slowly cooling, lifting the obtained crystal out of the liquid level and cooling to room temperature so as to obtain the NaSr3Be3B3O9F4 nonlinear optical crystal. The crystal has a nonlinear optical effect and wide light transmission band. Ultraviolet cutoff edge reaches 170 nm. The crystal doesn't resolve in water, has good chemical stability, is suitable for laser frequency conversion requirements of ultraviolet band, can be used to manufacture nonlinear optical devices and harmonic light output devices for realizing 2 octaves, 3 octaves, 4 octaves, 5 octaves or 6 octaves of Nd:YAG laser, and also can be used in harmonic light output devices of other laser wavelength to generate coherent light with its wavelength being equal to or less than 266nm for output.