Foundation pit construction method employing steel sheet piles as support cofferdam

The invention discloses a foundation pit construction method employing steel sheet piles as a support cofferdam. The foundation pit construction method is suitable to high-water permeability sand soil geology or high-flow plasticity sludge geology and includes the steps of A, inserting and driving the steel sheet piles to form the steel sheet pile cofferdam; B, performing the ground sealing to a bottom face of the foundation pit to be constructed by employing a high pressure jet grouting method; C, excavating the foundation pit and mounting an inner support of the cofferdam. Compared with the conventional method that the ground sealing is performed to the foundation pit after the excavation, the foundation pit construction method has the advantages that the excavation is performed before the ground sealing, thus cofferdam collapse and piping or upwarp of the cofferdam base never occur in the case of the high-water permeability sand soil geology or the high-flow plasticity sludge geology, and the building-around settlement, caused by drainage and rainfall, is avoided to prevent ground sealing under the water.