The bound water in soil directly controls the special physical, chemical, and mechanical properties of clay. This paper carries out isothermal adsorption tests on a series of artificially mixed clays and explores the relationship between bound water content and clay mineral composition (montmorillonite content). On this basis, three common empirical models of adsorption were adopted to fit the isothermal adsorption data separately, and their applicability to bound water adsorption by clay was evaluated in turn. The results show that by the isothermal adsorption method, the relatively humidity (RH) of 0.9 is the boundary between strong and weak bound waters, and the RH of 0.98 is the boundary between weak bound water and free water; For the mixed clays with the same mineral composition, the bound water content linearly increases with the growing content of montmorillonite; GAB model works excellently at RH = 0 ~ 100%, with a good fitting effect (R2>0.90); the parameters of GAB model have clear physical meanings, and change in line with the trend of the hydration process of bound water adsorption by clay. Therefore, GAB model is the most suitable model for fitting the isothermal adsorption data on bound water of clay.