Principal component analysis was used to study relationship between soil enzyme activity and soil fertility of paddy fields under the wheat-rice cropping system in a 25-year long-term experiment in the Taihu region.Results show that significant linear relationship was observed of soil fertility factors with the activities of urease,acid phosphatase,alkaline phospatase,β-glucosidase,and arylsulfatase,especially acid phosphatase and β-glucosidase,which showed significant or extremely significant linear relations with pH,available N,available P and organic matter.Analysis was carried out of the 14 treatments of the experiment and soil enzyme activities and fertility factors therein,and main factor groups affecting soil fertility were screened out for comprehensive evaluation.The evaluation reveals that soil enzyme activities may be cited as important indexes reflecting soil fertility,and phosphatase,arylsulfatase andβ-glucosidase are better than urease in this function.Combined app lication of N and P fertilizers is the optimal soil building mode.Addition of phosphate fertilizer in fertilization can increase soil fertility.No fertilization or application of organic manure only will lead to degradation of soil fertility.Straw incorporation can improve soil fertility in the wheat season,but has no obvious effect in the rice season.