Abstract: Cracks are an important influencing factor that leads to strength decay and permeability increase of expansive soils. A quantitative method for cracks positioned by closed curve is proposed based on the characteristics of crack development in expansive soils, and then the influences of the cracks on the strength and permeability of expansive soils are further investigated. The research results show that the method significantly reduces the relative error of the width of cracks, and the linear crack ratio avoids the processing problem of the influences of outer boundary shrinkage of samples on the crack development characteristics. Accordingly it improves the reasonability of the measured values of cracks. The evolution characteristics of cracks are enriched according to the variation of the linear crack ratio of different width grades. Further, a linear quantitative relationship among cracks, strength and permeability is revealed, and then the measured values of cracks can be used to replace the drying-wetting cycle times, which solves the problem that the drying-wetting cycle times are difficult to be quantified in practical engineering. Therefore, the quantitative crack parameters established by the closed-curve positioning method are more advantageous to the evaluation of the behaviors of expansive soils and the engineering application of crack researches than those by the area method.