More than 70 percent of all clays on the Earth's surface are of mixed-layer types, and knowing their swelling behavior is crucial to reducing soil expansion and mining development. Through the water-insert and interaction energy scripts, a molecular dynamics (MD) study of the differences in swelling behavior between the most important illite-montmorillonite (I-M) mixed-layer clays (MLCs) and Na-montmorillonite (MMT) is conducted. It captures the evolution of quantitative properties such as basal spacing d and interaction energy in the clay interlayer, increasing hydration for the first time. It is found that MLCs have smaller swellings than Na-MMT due to the asymmetric interlayer and mixed counterions. The clay-clay and clay-ion interaction energy drop, while the clay-water interaction energy increases with increasing hydration. In addition, the attractive interaction of clay-bound water seriously promotes swelling, of which the Coulomb interaction plays the most dominant role. The higher the K+ concentration, the more pronounced these phenomena are. This work provides insight into the molecular mechanism for initial swelling and clay-bound water interaction in MLCs. This will help to decipher its specific role in soils and minimize clay swelling in geohazards.