Soil samples for analysis were collected at different depths within 0~60cm from three plots different in irrigation mode in the same protected field where an irrigation experiment had been going on for 13 years to explore the effect of irrigation mode on characteristics of soil acidification. Analysis of the samples show that the soil active acidity and the content of exchangeable acid decreased with soil depth in all the treatments. Soil active acidity in the 0 ~ 40 cm soil depth varied sharply between the treatments, displaying a general order of furrow irrigation > subsurface irrigation > drip irrigation, while the differences in content of exchangeable acid between the treatments were significant mainly at the depth of 0 ~ 30cm, showing an order of subsurface irrigation > furrow irrigation > drip irrigation. Content of exchangeable Al³⁺ increased first and then decreased with increasing depth in all the three treatments, but the lowest in the treatment of drip irrigation. Soil BS increased with soil depth and showed an order of drip irrigation > subsurface irrigation > furrow irrigation in the 0~30cm soil depth. Soil pH was negatively related to exchangeable acid and nitrate nitrogen, but positively to base saturation, especially exchangeable Ca²⁺ saturation, both to an extremely significant extent. The proportion of Al³⁺ against exchangeable acid was negatively related to organic matter content, also to an extremely significant extent. In conclusion, soil acidification in protected fields is closely related to content of nitrate nitrogen, base saturation and content of organic matter. Compared with furrow irrigation and subsurface irrigation, drip irrigation is conducive to inhibition of soil acidification.