Abstract: In combination with the excavation process of the construction in Luwangfen test section, the analyses on causes and characteristics of swelling rock such as mineral chemical composition, dilatability, natural moisture content and density, strength and the engineering geological properties of expansive rock of the Middle Route of the South to North Water Diversion Project are performed. The results show that marl generally possesses potential in weak expansion, clay rock has weak-medium expansion behavior, of which particular rock body is strong expansive, and that the strength of the expansive rock will decline under the conditions of dry-wet alteration and expansion-shrinkage deformation. So, the volume of the rock will expanse after it absorbs water, and the canal lining will be damaged by the expansive force. From this study result, corresponding treatment measures should be adopted when the canal will be constructed.