The development from neutral (pH 6.5-7.5) to acidic (pH 4.5-5.5) is the key stage of soil acidification, so the buffering capacity of soil to acid in the range of pH 4.5-6.5 will directly affect the rate of soil acidification. In the study, 48 representative paddy soil samples were collected from plain areas of Zhejiang Province, and the acid buffering performance and its main influencing factors of the soils in the key pH stage of acidification were characterized. The results showed that the acid buffer capacity of the paddy soils, the amount of acid needed to reduce the pH value by one unit, ranged from 8.34-41.22 mmol/(kg·pH), with an average of 23.38 mmol/(kg·pH). The capacity decreased in the order of soils in river-net plain>soils in coastal plain>soils in river valley plain. The acid buffer capacity of the paddy soils in the river-net plain and coastal plain was 1.60 and 1.17 times of that in river valley plain, respectively. Correlation analysis showed that soil clay content and CEC were the main factors affecting soil acid buffer capacity, and the lack of clay content was the main inducement of soil rapid acidification in the plain area. Low-clay soil is the focus of farmland acidification prevention in the plain area.