Construction method for shallow overburden hard-based rock deep water deep foundation pit combined cofferdam

The invention discloses a construction method for a shallow overburden hard-based rock deep water deep foundation pit combined cofferdam. The construction method comprises the following steps of 1) preparation before construction, 2) cofferdam construction, 3) water stop layer construction, 4) preparation before hoisting and lowering of an inner supporting frame assembly, 5) lowering construction of the inner supporting frame assembly, 6) first inner supporting frame construction, 7) second inner supporting frame construction, 8) third inner supporting frame construction, and 9) fourth inner supporting frame construction. By adopting the construction scheme that a combined pile formed by combining a steel pipe pile and a steel sheet pile is adopted, the combined pile and the steel sheet pile are firstly inserted into a rock stratum surface to be fixed, then holes are drilled in the steel pipe pile, concrete is poured into the holes to form an anchoring pile, a water stop layer is manufactured at the junction