Field tests have been made in the expansive rock test section(Luwangfen Section) to study the failure mechanism of and the treatment measures for expansive soil canal slope, which is considered as a key technology of the South-to-North Water Diversion Project. Based on the test results, two failure modes of unstable canal slope are put forward according to the stability monitoring and protection survey. The effects of the treatment measures are comprehensively evaluated, and treatment plans for the canal slope are proposed. Reinforced geogrid is recommended first and foremost in the treatment ； while soilbag can be used at the outlet or twisted section where spot treatment is required ； and composite geo  membrane, as it has no weights effect, can be used at the canal slope above the first berm with low groundwater level and little lateral recharge.