Abstract: The stiff clay widely distributed on the bank slope of the project of “Diversion of water from Changjiang River to Huaihe River” is bound to cause a series of engineering problems under the continuous soaking of river water, which will have a great impact on the stability of the bank slope. In order to explore the stiff clay under continuous soaking strength degradation mechanism, taking the stiff clay of the bank slope of the Hefei section of the project of “Diversion of water from Changjiang River to Huaihe River” as the research object, and based on the continuous soaking test, triaxial compression test, mineral composition analysis test, microstructure test and ion concentration of soaking solution test, the strength of stiff clay under continuous soaking degradation mechanism was studied. The results show that the strength of stiff clay will deteriorate under continuous soaking, and the deterioration is obvious within 30 days, and the deterioration is weak and tends to be gentle after 30 days. The mass fraction of calcite in stiff clay decreases exponentially with the increase of continuous soaking time, and the Ca²⁺ concentration in soaking solution increases logarithmically. With the increase of continuous soaking time, the number and size of micropores on the sample surface increase, and the increase rate is fast within 30 days. After 30 days, the increase rate slows down and gradually stabilizes. The strength deterioration of stiff clay after continuous soaking is caused by water absorption and expansion of hydrophilic clay minerals and dissolution of calcite, which acts as the “skeleton” in the soil.