In this work, a fixed bed reactor was used to investigate the effects of the addition of P-based additives on the particulate matter (PM) emission characteristics from corn stalk combustion. The results showed that PM emissions from corn stalk showed a bimodal distribution with mainly PM1. The addition of these two additives could effectively reduce PM10 emission especially PM1 emission and the reduction was closely related to the molar ratio of P/K in the ash. For NH4H2PO4, PM1 and PM1-10 decreased firstly and then increased with increasing the molar ratio of P/K from 0.5 to 2, and reached its lowest value when the molar ratio of P/K was 1. For Ca(H2PO4)2, with increasing the molar ratio of P/K, PM1-10 decreased but PM1 decreased firstly and then increased. Under the current experiment condition, the optimal PM reduction effect was achieved when the molar ratio of P/K was 1. The addition of NH4H2PO4 increased PM1-10 by 22.46%~46.99%. Conversely, the addition of Ca(H2PO4)2 reduced PM1-10 by 13.71%~15.03% when the molar ratio of P/K was 1~2. The reaction between these two P-based additives with K salts in corn stalk ash, which effectively reduced the release of K to gas phase, was attributed to the reason of PM reduction. Generally, Ca(H2PO4)2 showed a better PM reduction compared to NH4H2PO4. This study could provide a scientific guidance for the mitigation of PM emissions during the combustion of corn stalk.