According to the characteristics of the expansive soil which are the expansion of soil meets water and shrinkage of soil loses water, the strength change law of shell modified expansive soil was studied by crushing the shells and adding them to the swelling soil through the scientific indoor shear strength test. The test results show that adding shells can effectively improve the shear strength of expansive soil. With the increase of the shell content, the shear strength increases first and then decreases, and the shear strength of the improved soil reaches the maximum when the amount of the shells is 12%.