context: Polylactic acid (PLA) is a type of bio-based, renewable, and biodegradable material made by starch raw materials from renewable plant resources. The physical properties of small molecules of poly(ethylene glycol) (PEG)–PLA were analyzed with FAMD simulations with the COMPASS forcefield, and the mechanical properties of large-chain PLC were studied using CGMD with the MARTINI force field. Recent research focused on designing PLA composites through MD simulations to improve the inherently weak mechanical properties of PLA. For example, MD simulations of the microstructures of PLA–o-carboxymethyl chitosan (CMC) composites showed that introducing CMC enhanced the stiffness of PLA, and the analysis of the interfacial properties of PLA–graphene composites yielded a scheme to design nanocomposites with high strength and toughness. Other than characterizing the molecular phenomena of polymers, the swelling behavior of cross-linked hydrogels in poor solvents were also studied using a CG model, where counterions were considered explicitly. Similar swelling conformations were found for charged cross-linked hydrogels as single polyelectrolyte chains. Furthermore, the effects of various experimentally-controlled parameters were systematically investigated to provide insight into the rational design of hydrogels with desired conformations and properties.