The solid-phase graft polymerization of hydroxymethyl acrylamide (HMA) onto polypropylene (PP) was realized by employing our self-designed pan-type milling equipment which has a unique and smart structure and can exert quite strong shear forces and pressure on the materials in between and break them down. When PP particles and HMA are pan-milled together, the macromolecular radicals generated from the chain scission of PP under stress can initiate HMA to polymerize, forming the PP-g-HMA graft copolymer. The graft copolymers were characterized by chemical titration, FTIR, DSC, and contact angle measurement. The amount of grafted HMA depends on the HMA concentration, increase of the PP particles' surface area during pan milling, temperature, as well as rotation speed of the mill pan. The percentage of grafting reaches 2.43%. The particle-size analysis showed that PP with a larger particle size favors the graft polymerization of HMA onto PP. DSC analysis demonstrated that the crystallinity of PP-g-HMA decreases as compared with PP due to the grafting of HMA onto PP.