The invention discloses a light-path system for a mode-locked ultraviolet laser. The light-path system comprises a pump light source, a resonant cavity unit, a beam splitter, a frequency doubling unit and an output unit, wherein the pump light source in a device is input into the resonant cavity unit; a generated 1064nm mode-locked laser light enters the frequency doubling unit, and is converted into a 355nm laser light through double-frequency and triple-frequency parameters of lithium triborate crystal; and finally the output unit carries out light beam quality modulation to obtain a high-quality mode-locked ultraviolet laser light. With neodymium-doped yttrium vanadate crystal as a working material, the neodymium-doped yttrium vanadate crystal is excellent in performance, has a relatively high absorption coefficient and a relatively large stimulated-emission cross-section on a pump light, and can reach relatively good frequency-doubling conversion efficiency when used cooperatively with the lithium triborate crystal. Meanwhile, a mode-locked self-starting mechanism is provided by a semiconductor saturable absorber mirror; the mode-locked ultraviolet laser can be manufactured; and the laser light is subjected to spontaneous parametric down conversion to obtain an entangled state of the orbital angular momentum after passing through BBO crystal, and can be applied to preparation of orbital angular momentum entanglement in the field of quantum communication.