Abstract: Co-pyrolysis of biomass wastes can be expected to serve as an efficient conversion of biomass resources and the recycling of degradable materials, particularly with the urgent global demand for sustainable and carbon neutrality. It has been of great significance for waste management in recent years. Renewable energy can also be integrated to realize sustainable agriculture. This study aims to explore the co-pyrolysis process of microalgae and polylactic acid after aging. Meanwhile, a systematic investigation was conducted to clarify the synergistic effect of the pyrolysis reaction, kinetics, and products of the microalgae and polylactic acid blends with different mixing ratios. The polylactic acid aging treatment was also performed on the co-pyrolysis process with microalgae. Five mixtures of microalgae and polylactic acid were prepared, namely 100% microalgae, 75% microalgae + 25% polylactic acid, 50% microalgae + 50% polylactic acid, 25% microalgae + 75% polylactic acid, and 100% polylactic acid. PLA was placed in a 120 ℃ water bath to simulate aging for 12, 24, and 96 h. After aging, the PLA was mixed with microalgae in a 1:1 ratio. Pyrolysis experiments were conducted on the blends of microalgae and polylactic acid, the aged polylactic acid samples, and the blends of aged polylactic acid and microalgae. A series of experiments was carried out using the TG-FTIR-MS system. The temperature was heated from 40 ℃ to 900 °C at the heating rates of 5, 10, and 20 ℃·min^-1 under a nitrogen atmosphere. The thermogravimetric test was carried out to determine the weight loss stage, characteristic temperature, and thermal stability of the pyrolysis. The apparent activation energy was calculated by using the kinetic models (KAS, FWO). The types and evolution of gaseous volatile products released by pyrolysis were monitored and then characterized online by FTIR and MS. The experimental results show that the mixing ratio had a significant influence on the pyrolysis behavior and kinetics. The TG and DTG indicated that the initial co-pyrolysis temperature and the thermal stability of the blend decreased with the increase of the proportion of microalgae. The co-pyrolysis shared the synergistic behavior, because the polylactic acid melt encapsulation delayed the residence time of volatile matter and the free radical reaction. Kinetic analysis further indicated that the co-pyrolysis significantly reduced the outstanding activation energy. There was the lowest average activation energy of the sample with 75% polylactic acid +25% microalgae during co-pyrolysis. The infrared spectroscopy and mass spectrometry show that the H₂O and CO₂ were significantly enhanced during co-pyrolysis, with the increase in the proportion of microalgae. Peak release temperature was shifted to the low-temperature region. The proportion of polylactic acid was enhanced C=O, for the inhibition of CH₄. Furthermore, the pyrolysis experiments were carried out on the polylactic acid with different aging times. It was found that the polylactic acid samples were the most outstanding after 96-hour simulated aging. The thermogravimetric results indicate that the simulated aging reduced the content of ester groups in the polylactic acid molecular chain. The PLA aging was blended with microalgae more prone to pyrolysis. There was the most outstanding synergistic behavior of the blends when the aging time of polylactic acid reached 96 h. In conclusion, there was a significant effect of the mixing ratio of the microalgae and polylactic acid and the aging state of polylactic acid on the co-pyrolysis characteristics with microalgae. Energy consumption was reduced to combine the physical and chemical synergistic mechanisms. The finding can provide some perspectives for the resource utilization of wastes and the low-carbon technologies. The pyrolysis process was also optimized to reduce the energy consumption, indicating the high efficiency of waste treatment. This finding can provide a theoretical basis for the co-pyrolysis process of polylactic acid and microalgae with various aging degrees.