Abstract: In order to reduce N-/O-compounds content and improve the quality of microalgae bio-oil, the co-pyrolysis/catalytic of Nannochloropsis sp. (NS) and polyethylene (LDPE) were studied in a fixed bed, and the distribution of N and O was discussed, as well as the interaction between microalgae and LDPE and the influence of the addition of catalyst. It was found that co-pyrolysis could effectively inhibit the transformation of O and N to oil, and promote the O release as H₂O and N conversion to gas products. In addition, plastic adding significantly reduced the content of O-/N-compounds in oil, such as carboxylic acid, amide and N-heterocyclic, and increased the aliphatics content. Besides, it effectively promoted the formation of hydrocarbon gas, and showed a certain synergistic effect on CO and H₂, and the interaction reached the maximum at 25%LDPE. Furthermore, ZSM-5 could promote the formation of hydrocarbon gas, increase the LHV of gas products (35.6 MJ/Nm³), and further reduce the nitrogen compounds in the oil, while the N in microalgae transferred to gas, and O converted to gas and H₂O, which resulted in the further reduction of the O and N contents in the oil. Moreover, catalytic co-pyrolysis could inhibit the formation of aromatic hydrocarbons to a certain extent and improve the selectivity of aliphatic hydrocarbons.