Slope protection is necessary for the deformation control and stability of the expansive soil slopes by the main canal of the Middle Route of South-to-North Water Transfer Project. After analyzing the project background and the properties of expansive soil, the target of slope protection is suggested to be set to control the water content change and to restrain the crack for expansive soil. The defects of the slope protection scheme only with waterproof function are pointed out. On the basis of the theory of saturated-unsaturated water flow, a protection scheme of double layers with water drainage function is proposed. Taking advantage of the law that the comparative relationship of permeability between unsaturated coarse and fine soil would switch over, this kind of scheme would achieve the aim to omnidirectionally control the water content change for expansive soil slope. The more important is that this kind of scheme would be efficient for long time.