The impact of rapid economic development on soil pH is obvious in peri-urban areas. A case study of a peri-urban area of Wuxi was conducted to study spatio-temporal variation of soil pH in the area and explore its relationship with soil chemical properties and its affecting factors. Results indicate that soil pH varies significantly (3.74～8.50). When the field changed from paddy field into vegetable garden, its soil became acidic with mean pH value decreasing from 6.43 to 5.40. Soil pH also decreased significantly in soils around industrial enterprises, while it increased in soils around towns and alongside highways. Soil acidity is determined jointly by exchangeable H⁺and Al³⁺. Their appearance in vegetable plots led to reduced total exchangeable base and base saturation degree, and hence lower pH in the soil. Comprehensive analysis shows that agricultural activities, including change of land use, continuous intensive utilization and heavy application of inorganic fertilizers, were major factors affecting soil pH, while construction of highways and towns bringing in alkaline substances to the soil followed in contribution. Therefore, higher organic manure application rate and paddyupland rotation is suggested for the study area.