Abstract: According to quantitative study results of soil structure， nine structure indexes of granulometric composition， aggregate and porosity are used to establish structure synthetical evaluation system. Black soil structure is synthetically evaluated by the means of gray fuzzy clustering method and the structure quality grade and fertility situation of Jilin black soil are determined. Results show that soil with moderate-low fertility quality occupies 53% of Jilin black soil， which demonstrates that there is a worse fertility condition of Jilin black soil. It doesn’t need to establish classifying standard in soil structure evaluation using gray fuzzy clustering method，and the best clustering analysis can be done according to the actual condition of evaluated objects. It is feasible that structural parameters are used in the evaluation of black soil fertility and degradation.

Key words: black soil, structural synthetical evaluation, water-stable aggregate, gray fuzzy clustering, Jilin Province