Fly ash particles, generated during the operation of biomass power plant boilers, deposited on the surface of metallic heating components are likely to cause metal thinning or corrosion leakage, which is an important cause for the frequent occurrence of corrosion and tube-bursting accidents of the metallic heating components for the power plant boilers. Herein, the effect of particles of different sizes (< 30, 30-75 and 75-100 μm) and different deposit amount (10, 20, and 30 mg/cm²) of pine wooden chip ash on the surface of 12Cr1MoVG steel, a typical common steel used for boilers of biomass power plants in the country, on the high-temperature corrosion behavior of the steel was investigated via a home-made set of a tube-furnace system to simulate typical boiler flue-gas composed of CO₂, O₂ and N₂ at 600 ℃, as well as per weight change measurement and relevant characterization techniques for corrosion products, in terms of the corrosion mechanism of the boiler tube steel induced by fly ash deposits generated during wooden biomass combustion. The results show that the influence of wooden ash particles on the corrosion of the boiler tube steel surface is determined by the particle size of the fly ash and its deposition amount. Namely, for the cumulative ash deposition amount is 10 mg/cm² and their particles range within 75-100 μm (large particles), due to their relatively small specific surface area, they are not prone to agglomeration, therewith the ash deposits present a loose and pore structure, which may be conductive to the continuous release and migration of certain active substances and promotes the localized corrosion; For the particles of size range within 30-75 μm, and the ash deposit amount of 20 mg/cm², the hierarchical pore structure of the deposit may be conductive to accelerated corrosion, when the cumulative ash amount increases to 30 mg/cm², the appropriate gradation of particles of different sizes leads to a decrease in the porosity of the deposits, thereby acting as a corrosion barrier. It follows that the most serious corrosion occurred for particles range within 75-100 μm while the ash deposit amount of 10 mg/cm², the corrosion degree is the next for particles range within 30-75 μm while the ash deposit amount of 20 mg/cm², in contrast, the corrosion degree was also the most significant when the particle size below 30 μm while the ash deposit amount of 20 mg/cm². The findings can provide a reference for understanding the characteristics and mechanisms of high-temperature corrosion of boiler tubes in biomass power plants induced by of different sized biomass fly ash.