Abstract: In the hydropower and water conservancy projects, the quality of contact soil is usually judged by the plasticity index and clay content. The plasticity of the soil depends on the type and content of highly hydrophilic clay minerals in the clay particle, which are capable of physicochemical interaction with water. The greater the plasticity index, the worse the water stability of the soil. Through analysis, the definition of the plasticity index and clay content of the contact soil in the current standard is not reasonable. The plasticity index I_p=10~26 and the activity index A_c=0.75~1.25 are proposed as the quality control indicators for contact soil materials. The clay content of soil materials for the Miaowei hydropower station is lower than the requirements of the specification, but the activity index A_c is calculated to be 1.08~1.25, which meets the requirement of activity index, so that it can be used as contact soil. After the Miaowei hydropower station was put into operation, according to the monitoring data, the displacement of the contact clay soil on both sides of the bank was small and no plastic failure occurred, which proved that the proposed control index is feasible.