Biomass has been one of the most promising renewable energy substitutes for fossil fuels. However, the bio-oil production from biomass pyrolysis has been greatly limited the application value, due to the complex components and low enrichment of available components. An effective way can be to selectively regulate the pyrolysis reaction using the catalyst. Among them, the coal fly ash can be widely expected to serve as the low-cost and high-efficiency catalytic medium, due to the rich pore structure and inorganic mineral composition. Generally, solid waste is produced by the combustion of pulverized coal in the boiler of the thermal power plant. More importantly, CO2 rich in carbon source can participate in the pyrolysis process to enrich the platform compounds. Therefore, CO2 and coal fly ash can be applied for the biomass pyrolysis process, in order to improve the quality of bio-oil for the high-value utilization of solid wastes. It is very necessary to explore the co-pyrolysis behavior of coal fly ash and biomass in the CO2 atmosphere for the goal of carbon neutralization. This work aims to investigate the effects of coal fly ash on the distribution of the products during the pyrolysis of maize stalks with different particle sizes. The liquid products were also characterized by catalytic pyrolysis under N2/CO2 atmosphere. The coupling mechanism of CO2 and coal fly ash was proposed, according to the experimental data. Specifically, the particle sizes of corn straw were ranged from 0.1-0.16, 0.17-0.21, 0.22-0.30, and 0.31-0.45 mm, respectively. The mixing ratio of maize stalk and coal fly ash was 1:1. A catalytic pyrolysis experiment of maize stalk was carried out in a fixed bed reactor, where the temperature was 550±1 ℃, the flow rate of N2 was 100 mL/min, and the flow rates of CO2 were 40, 70, 100, and 130 mL/min. The obtained bio-oil was detected using an elemental analyzer and Gas Chromatography-Mass Spectrometry (GC-MS). The results showed that the particle size dominated the heat and mass transfer behavior of the biomass pyrolysis process. The increase of particle size led to the increase of bio-oil production first and then the decrease in only single biomass pyrolysis. The yield of semi-coke decreased, but the gas increased. The addition of coal fly ash was conducive to the formation of bio-oil, where the maximum bio-oil yield was 44.32% using 0.17-0.21mm of biomass particles. Coal fly ash was also greatly contributed to the biomass pyrolysis with the large particle size, where the bio-oil yield increased by 8.94 percentage point, compared with the blank sample. The main reason was the synergistic effects among metal components in the coal fly ash, thereby enhancing the heat and mass transfer efficiency inside and outside biomass particles. In addition, CO2 was introduced to inhibit the formation of bio-oil, and the liquid yield decreased with the increase of CO2 flow rate in the system. The co-pyrolysis of coal fly ash and biomass with small particle size was conducive to enhancing the selective phenolic compounds. The strong binding ability of CO2 and hydrogen radical was further promoted to enrich the phenolic compounds in the system. The methoxyphenol compounds increased significantly among phenolic compounds in the bio-oil. The relative content of phenolic compounds was the highest- when the flow rate of CO2 was 40 mL/min. [ABSTRACT FROM AUTHOR]