Construction method for steel sheet pile cofferdam in complex geology

The invention provides a complex geology steel sheet pile cofferdam construction method which comprises the following steps: surveying and setting out; steel sheet piles are inserted and driven through the steel casings until a cofferdam structure is formed by surrounding and folding; mucky soil in a sludge area in the cofferdam structure is removed, and underwater bottom sealing pouring is conducted on the sludge area for the first time; after the first-time bottom sealing pouring reaches the design strength, layer-by-layer water pumping operation is conducted, and meanwhile a plurality of steel enclosing purlins and inner supports are installed layer by layer from top to bottom; after all the inner supports are installed, ground straining beams are excavated and poured in a segmented mode on the hard soil area in the cofferdam structure, remaining hard soil is excavated, secondary dry operation bottom sealing pouring is conducted on the hard soil area, and the construction procedure of bottom sealing with h