The forming property of clay is one of the most important processing properties. Mineral composition and plasticity of various kinds of clays were determined, and the relations among yield value, degree of deformation and mineral composition were analyzed. According to the relations between acting force and distance between colloidal particles in clay-water system, the determining method of plasticity index II was set up, and compared with the traditional determining methods. The results show that the determining method of plasticity index II can better reflect the effect of clays on the plasticity of body by the comparison of determined values in different ways with the actual plasticity of body, the plasticity indexes II of different kinds of clays have relations of summation with the plasticity of body. The forming property of clay can be reasonably evaluated by plasticity index II.