In this study, intumescent flame retardant polylactide/trisilanolisobutyl polyhedral oligomeric silsesquioxane (PLA/TPOSS) composites were prepared via melt blending method. The morphology of the PLA nanocomposites was assessed by transmission electron microscopy (TEM). The flame retardancy of the PLA/TPOSS composites was evaluated by the limiting oxygen index (LOI) test and the UL-94 test. It was observed that the addition of TPOSS had evident flame retardant effect in comparison to the results obtained from neat PLA. The PLA nanocomposite showed a 69% reduction of heat release capacity (HRC) when tested in the microscale combustion calorimeter (MCC). Thermal gravimetric analysis (TGA) showed the incorporation of intumescent flame retardant, and TPOSS had effectively delayed the thermo-oxidation process of the PLA. The thermal degradation mechanisms of the PLA/TPOSS composites were investigated by thermogravimetric analysis/infrared spectrometry (TG-IR) and direct pyrolysis/mass (DP-MS) analysis.