The invention relates to the technical field of underwater foundation construction of bridges, and particularly discloses a construction method of a steel sheet pile cofferdam inner support and drilling pile steel casing installation construction platform. Construction is conducted in the short-time low-water-level period of a riverway and comprises the following steps that short building island construction is conducted, a foundation pit is excavated, and circular grooves and a rectangular groove are excavated at the bottom of the foundation pit; steel casings are installed in the circular grooves, and inter-pile connecting systems and brackets which serve as installing platforms are connected among the steel casings; steel sheet pile cofferdam is installed in the rectangular groove to aclosure, and a concrete water-stop belt is poured on a formwork arranged on the outer side of the top of the rectangular groove; and then steel casing heightening is conducted on the installing platforms, the installing platforms are disassembled and assembled to be moved upwards, finally the installing platforms serve as construction drilling platforms, and construction is completed. The installing platforms can be cyclically disassembled and used and not only can be used for steel sheet pile cofferdam inner support installing, steel casing heightening and drilling platform assembling, but also can enhance the steel casing connecting stability, disassembling and assembling are convenient and fast, the construction period is effectively shortened, construction input is reduced, and the cost is reduced.