Poly(lactic acid) (PLA)/ diisopropylphenyl peroxide (DCP) blends were prepared by melt blending method using DCP as a free radical initiator, and the effects of the free radical initiator DCP on the gel content, crystalline properties, rheological properties and mechanical properties of PLA were investigated. The mechanical tests and dynamic rheological experiments, combined with Han and Cole-Cole plots, reflect that the DCP-modified PLA has cross-linking and branched structures, and the formation of such cross-linking and branched structures improves the tensile strength of PLA, while significantly increases the complex viscosity, energy storage modulus and loss modulus of PLA. The modified PLA has a longer relaxation time and a more pronounced elastic response, which effectively improves the melt strength of PLA. When the mass fraction of DCP is lower than 0.3%, only a small amount of gel is produced, and the formation of this cross-linking and branched structure can promote the formation of crystalline nuclei, which plays the role of heterogeneous nucleation, effectively improves the rate of crystallization of PLA and increases the degree of crystallinity.