Feedstock recovery plays a crucial role in the further application of polylactic acid (PLA), yet it still lacks efficient ways, resulting from an unclear understanding of PLA pyrolysis. Hence, the co-pyrolysis of PLA and the major components of biomass additives cellulose, hemicellulose and lignin was carried out to enhance the feedstock lactide yield and explore the pyrolysis mechanism. Compared to cellulose/PLA and hemicellulose/PLA, the co-pyrolysis of lignin/PLA had the strongest synergistic effects. Based on in situ FTIR and Py-GC/MS, the promoting mechanism was as follows: 4-ethyl-2-methoxy-phenol produced by the pyrolysis of lignin provided additional free radicals for the free radical reaction of PLA so that the production of lactide increased by 162.8%. The activation energy of co-pyrolysis was 30.6% and 13.8% lower than those of lignin and PLA, respectively, which was also due to the fact that lignin promoted the free radical reaction of PLA. It was indicated that co-pyrolysis required less energy than that of single pyrolysis. In addition, the generated lactide was not well-compatible with other bio-oils and solidified at room temperature, so it could be recycled by simple physical separation. This work provided a new method for the reuse of large amounts of waste PLA and lignin waste such as papermaking black liquor.