We demonstrate high-efficiency ultraviolet (UV) laser generation in a nonlinear optical crystal $\mathrm{Ba}\mathrm{Al}\mathrm{B}{\mathrm{O}}_3{\mathrm{F}}_2$ (BABF); $355\mathrm{nm}$ UV light was generated by using a type I BABF crystal as a sum-frequency mixing of the fundamental light and the second harmonic from a picosecond Nd:YAG laser operated at a repetition rate of $10\mathrm{Hz}$ with a pulse duration of $20\mathrm{ps}$ . An output of $0.635\mathrm{mJ}$ for the third harmonic was obtained. The conversion efficiency was 26.4%. As far as we know, it was the first time that third-harmonic generation in BABF was achieved. Obviously, the BABF crystal is a promising UV nonlinear optical material because of its relatively large nonlinear coefficient and nonhygroscopicity.