Because expansive soil is very sensitive to water,the stability of expansive soil slope and its lining structure is one of the important challenges for the construction and operation of the first-stage project of middle route South-to-North Water Diversion in China. On the basis of geological structure classification, the permeability structure and seepage field system in expansive soil area and also the seepage field system in expansive soil slope of canal were classified respectively. For different kinds of seepage field system in expansive soil slope of canal, the seepage field transformations caused by canal construction were analyzed. The main tasks for seepage field control were discussed, and the principle was pointed out for the application of seepage field control scheme: laying drainage liner for long-term control of water content in unsaturated slope, deliberate design of berms with their negative effects on seepage control considered, reasonable utilization of drainage holes and relief wells to control seepage field and ground water pressure, reasonable arrangement of sensitive and reliable check valves for the canal stretches with the variation range of ground water level overlapping that of canal water level, tracking research on the efficiency of seepage field control and timely regulating it effectively.