Abstract: Evaluation of swelling-shrinkage grade for untreated and treated expansive clay is a complex system problem since it involves the uncertainty of human thinking, multiple types and variations of influence factors. Based on the interval number theory and coupling analytical method of set pair analysis, evaluation indices and classification standard are first described in interval numbers. Then connectional expectation and an analytical model are proposed to interpret the certainty and uncertainty, and evolvement trend of swelling-shrinkage grade in a unified way. The results from practical application and comparison with other methods show that the proposed model is feasible and reliable, and simplifies the analytical process of interval number relationship. It also serves as a reference method for other similar problems.