The emission of ultrafine particulate matter (PM0.2) originated from the agricultural biomass pellet combustion poses great threat to atmospheric environment and human health, which restricts its large-scale utilization. In this study, a new phosphoric acid modification method is proposed to improve the PM0.2 reduction efficiency by kaolin additive. The effects of phosphoric acid concentration and treatment time on the physicochemical properties of kaolin and on the mitigation of PM0.2 emission from the pellet combustion are investigated. Results indicate that phosphoric acid modification destroy the internal structure of kaolin by the leaching of Al cations and the formation of active free silica. Meanwhile, the pore structure increases after modification with residual P deposited on the surface, which results in better alkali capture ability of modified kaolin. With the addition of phosphoric acid modified kaolin, significant reduction of PM0.2 emission can be achieved and the reduction ratio is proportional to the acid concentration. The maximum PM0.2 emission reduction ratio reaches 64.5% for the kaolin additive modified by 12 mol/L phosphoric acid for 6 hours. Finally, the PM0.2 reduction mechanism is proposed based on the analysis results, which provides technical knowhow for the industrial application of agricultural biomass pellet combustion.