Soil samples were collected from protected fields in Yuhong District of Shenyang, Liaoning Province, China, for determination of exchangeable acid, exchangeable base and pH, and further analysis of acidification characteristics of the soils and relationships of soil pH with the other two. Results show 1) that soil acidification displayed a clear rising trend after the field began vegetable cultivation under protection. Soil exchangeable acid content in the protected fields were much higher than those in the control. Soil pH was determined mainly by exchangeable acid in potential acid. The proportion of exchangeable Al³⁺ increased with exchangeable acid，while decreased with increasing OM. And, the proportion of H⁺ in exchangeable acid showed a reverse trend2）Exchangeable bases, especially exchangeable Mg²⁺, K⁺ and Na⁺, in various layers of the soils in the protected fields were much higher than those in the control, not only in content, but also in saturation, whereas soil exchangeable Ca²⁺ did not vary much, but the base saturation and exchangeable Ca²⁺ saturation decreased significantly3）Soil pH was in significant negative correlation with exchangeable acid and nonexchangeable acid，but in significant positive correlation with base saturation, which is mainly restrained by dominant exchangeable Ca²⁺.