Abstract: The hazards caused by the swelling and shrinkage due to the changing water content of expansive soil are common occurrences． Therefore it is of great importance to investigate the improvement of expansive soil． The chemical improving process is one of the common methods． Carbide slag is the main waste of producing acetylene industrially． Stacking the carbide slag is both land-consuming and pollution． This paper investigated the expansive soil improved with carbide slag through indoor experiments． The effect of carbide slag on the physics and mechanics characteristics was evaluated as well as the swell-shrink characteristics of expansive soil． The results indicated that the optimum moisture content increased and the maximum dry density decreased with the increase of carbide slag content． The plastic index of expansive soil increased at first and then decreased with the increase of carbide slag content; the free swelling ratio and the swell capacity and the swelling pressure and the linear shrinkage ratio were decreased with the increase of curing time． The compression modulus was increased with curing time，while the coefficient of compressibility on the contrary; the increasing of shear strength with curing time was largely reflected in the increasing of cohesive force，internal friction angle and unconfined compressive strength with curing time; the 10% carbide slag was the optimum mixing proportions． The scaning result by SEM verified that the strength of the soil increased with curing time，and the swell-shrink characteristics decreased with curing time．