Free-radical reactions of polymers, including biodegradable polymers via reactive extrusion, are normally induced by peroxide chemicals, which are known to lead to the formation of secondary products and impart some performances to the resin. Here, we report an ultraviolet (UV)-induced reactive extrusion, without employing a peroxide initiator, to control chain scission and branching reactions of polylactide (PLA). Through this technique, chain scission reaction of molten PLA induced by UV irradiation during extrusion was promoted to high-level efficiency. Degraded PLA samples had lower complex viscosity and storage modulus, because of random main chain scissions. Long-chain branched (LCB) structure of PLA was obtained when a multifunctional chemical agent, trimethylolpropane triacrylate (TMPTA), was added into the PLA matrix during extrusion. Various rheological plots including Viscosity, storage modulus, loss tangent, and Cole-Cole plots were used to distinguish the LCB structures of PLA samples. Thermal and crystallization properties of degraded and branched PLA samples were also investigated by means of differential scanning calorimetry (DSC) and polarized optical microscopy (POM). For the LCB PLA samples, a distinct crystallization exothermic peak appeared and accompanied by the disappearance of the cold crystallization temperature, demonstrating significantly enhanced crystallization rates. This UV-induced reactive extrusion has nonresidues of peroxide, is highly efficient and easily adjustable, and opens new avenues in potential applications for PLA modification, such as grafting and polymerization.