Effect of fertilization and straw application on soil acidification has been one of the foci in research of agricultural management and soil acidification. Based on a longterm fertilization experiment in paddy soil in the Taihu Lake region, China, effect of the longterm (26 years) fertilization on acidification of its topsoil (0～15 cm) was studied under a rice/wheat rotation system. The experiment was designed to have six treatments, i.e. (1) no fertilizer (control, C0), (2) pig manure alone (M0), (3) urea alone (CN), (4) urea combined with pig manure (MN), (5) urea combined with rice straw (CRN), and (6) urea combined with pig manure and rice straw (MRN). After 26 years, significant effects, different between treatments, were observed on soil pH and acidification buffering capacity. Treatment M0 was lower than Treatment C0 in pH but higher in acidification buffering capacity. Treatment CN and Treatment CRN was lower than Treatment C0 by 0.45 and 0.66 in pH, and by 1.52 and 0.95 mmol kg^-1 in acidification buffering capacity, respectively In terms of soil acidification rate, ranging from H⁺ 0.61 to 1.74 kmol hm^-2 a^-1 among the treatments, Treatment CN was higher than Treatments C0 and M0, but lower than Treatments MN, CRN and MRN, and Treatment MRN was the highest Correlation analysis revealed that soil pH were significantly related to input rate of exogenous nitrogen, soil total nitrogen level and available nitrogen level, which indicates that soil acidification is mainly related to nitrogen in the chemical fertilizer, manure and rice straw. However, the effects of urea differed from that of manure and rice straw on soil acidification. The former decreased both soil pH and soil acidification buffering capacity, while the latter maintained or slightly increased the capacity. Obviously long-term application of urea significantly stimulates soil acidification, and combined application of urea with rice straw does even more.