Abstract: Plastics as co-feeding can effectively improve the quality of microalgae pyrolysis oil. The effects of co-pyrolysis of microalgae and polyethylene (PE), polystyrene (PS) and polypropylene (PP) on pyrolysis characteristics and kinetics were investigated by TG-FTIR. There is an interaction between microalgae and plastic, which increases the pyrolysis temperature of plastic and inhibits the formation of coke. PE can effectively reduce the residual yield, while the pyrolysis temperature of PS obviously transfers to high temperature during co-pyrolysis. In addition, co-pyrolysis reduces the average activation energy, especially low proportion of plastic. Besides, the co-pyrolysis of microalgae with PE promotes C−C fracture to release CO₂ and −CH₃ and C=C−H. However, the co-pyrolysis of microalgae and PP inhibits the release of CO₂, and intensifies the formation of aromatic hydrocarbon C−H from polypropylene. Moreover, the co-pyrolysis of microalgae and PS can slightly promote the formation of C=O and NH₃, and aggravate the hydrogen transfer of microalgae to break aromatic ring of polystyrene and release CH₃.