The lateral deformation of the steel sheet pile cofferdam increases sharply during the construction of cement soil mixing piles inside the steel sheet pile cofferdam in a ship lock project in South China. In order to prevent the influence on the subsequent construction, this paper uses the method of numerical simulation and field monitoring to analyze the deformation rule and the cause. The construction method of stiffening cement mixing pile by inserting H-shaped steel into cement mixing pile is put forward. The results show that the construction method can effectively reduce the deformation influence of cement mixing pile construction on the surrounding steel sheet pile, ensure the construction safety, and provide guidance for the next step of cement mixing pile construction.