Construction method of combined cofferdam suitable for weak geology deep foundation pit

The invention discloses a construction method of a combined cofferdam suitable for a weak geology deep foundation pit and relates to the field of bridge construction. The construction method includes the following steps that the construction position of the concrete cofferdam is set out and positioned; base replacement is conducted on the construction position; supporting sole timber and steel blade feet are laid, rebars are tied, the concrete cofferdam is poured, and anchor rebars are installed on the top face of the concrete cofferdam; a first inner support is installed in the concrete cofferdam; the concrete cofferdam is made to sink; steel plate piles are installed on the top face of the concrete cofferdam, a second inner support is installed in the steel plate piles, the second inner support is fixed to the anchor rebars, and a third inner support is installed above the second inner support; the concrete cofferdam is made to sink continuously to design elevation. The combined cofferdam can meet the structural stress requirement; environmental pollution of construction waste, dust, noise and the like generated when the concrete cofferdam is detached is avoided; green construction is facilitated; construction progress is effectively accelerated; the construction cost is saved.