Currently, severe soil acidification occurs widely in cropland. Extensive application of acidic compound fertilizers in agricultural production may contribute to soil acidification, while its acidity characteristics and effect on soil acidification is unclear. In this study, 12 compound fertilizers with ternary nitrogen, phosphorus, and potassium elements were chosen from domestic and foreign companies due to their mass application in China. Results indicated that 11 compound fertilizers were acid, the fertilizers with pH between 4.0 and 5.0 accounted for more than 58%, and their acid content was up to 1.95 mol/kg. When these acid fertilizers directly applied to the soil, they could significantly reduce soil pH and increase the content of exchangeable acidity in one day. Therefore, application of acid compound fertilizer could significantly promote soil acidification due to their high acid content. When these compound fertilizers were applied to red soil for incubation, the ratio of protons from fertilizer direct input to nitrogen conversion could reach up to 204.6% on the 5^th day of incubation, while the maximum ratio was only 4.4% on the 60^th day of incubation. Results suggested the relative contribution of direct proton input to hydrogen ions by applying strong acid compound fertilizer was greater at the early stage of fertilizer application, while proton production from nitrification played a major role in the later stage. However, the contribution of long-term and mass application of acid compound fertilizer to soil acidification should not be ignored.