Intumescent flame-retardant poly(lactic acid)/starch (PLA/starch) biocomposites were prepared by means of melt blending. Microencapsulated ammonium polyphosphate (MCAPP) was added to the PLA/starch biocomposites not only to improve its flame retardancy but also to restrain the reaction between ammonium polyphosphate and starch during processing. The flame-retardant properties of PLA/starch biocomposites were evaluated by limiting oxygen index, UL-94 test, and microscale combustion calorimetry (MCC) test. The results of MCC showed that the peak of heat release rate and total heat release of PLA/starch biocomposites decreased dramatically compared with those of pure PLA. The thermal degradation and gas products of PLA/starch/MCAPP systems were monitored by thermogravimetric analysis and thermogravimetric analysis infrared spectrometry. Scanning electron microscopy and X-ray photoelectron spectroscopy were utilized to explore the surface morphology and chemical components of the char residues.