The invention belongs to the field of polymer materials, and discloses a preparation method of PP-g-MAH (Polypropylene grafting Maleic Anhydride) triggered and melt by ultraviolet irradiation. The preparation method comprises the following steps of (1) uniformly mixing polypropylene resin, MAH, a photosensitizer and a grafting additive; (2) setting the temperature of an extruding machine, and turning on an ultraviolet lamp, wherein a light passing opening is formed in a feed cylinder of the extruding machine, and the ultraviolet lamp is arranged above the light passing opening; (3) adding a mixture obtained in step (1) into the feed cylinder of the extruding machine; (4) turning on the extruding machine to carry out melt blending after the extruding machine reaches a set temperature, and obtaining the PP-g-MAH. According to the preparation method disclosed by the invention, the reaction time is short, and the equipment is simple; the reaction degree can be controlled by controlling the factors of the rotating speed of a main machine, the feeding speed, the temperature and the like; severe degradation of peroxide initiators on polypropylene can be avoided by using ultraviolet trigger.