The change tendency of paddy soil pH was analyzed by using long-term monitoring data of farmland in Guangdong Province, South China. The results showed that the overall variation of paddy soil pH was represented as acidification since 1984,with the average pH value decreased by 0.33 units, and the frequency distribution of strongly acid and acid soil showed a rising trend, increased by 3.2% and 21.9% respectively. The average pH value decreased by 0.54 units in the Pearl River Delta of Guangdong Province, significantly higher than those in the other regions. A single application of chemical fertilizer decreased soil pH value, but mixed application of chemical and organic fertilizers could improve the ability of soil to resist acidification, and improved paddy soil pH value. Heavy application of chemical fertilizers and ignoring the application of organic fertilizer is the main reason for the decline of the pH value of the paddy soil, and increasingly serious acid rain is the important reason for the decline of the soil pH value. We should pay attention to the use of organic fertilizer in the future for the improve-ment of soil acidification.