The invention discloses a construction method of an island-building steel sheet pile cofferdam type bored pile platform suitable for rock formation geology. The construction method of the island-building steel sheet pile cofferdam type bored pile platform suitable for the rock formation geology comprises construction in a first stage in which the riverway is in a low-stand period and constructionin a second stage, wherein the construction in the first stage comprises the steps of manually building an island, excavating a foundation pit and pumping water; mounting a steel sheet pile concrete locating block with paired guide posts at the bottom of the foundation pit; welding internal and external rotary roads consisting of I-steels; sleeving internal and external locating steel pipes on theinternal and external guide posts; welding a limiting steel ring, a bottom course waling and an internal support; mounting a steel sheet pile to be closed; after connecting and fixing the steel sheetpile with the bottom course waling, dismantling an internal locating steel pipe; mounting a first section of steel casing and assembling a joint system; dryly sealing a cushion layer; and dismantlingan external locating steel pipe and a limiting ring beam on the external locating steel pipe; and the construction in the second stage comprises the steps of mounting other layers of walings and internal supports, joining the first section of steel casing, and erecting a bored pile platform to finish the construction. According to the construction method of the island-building steel sheet pile cofferdam type bored pile platform suitable for the rock formation geology, by using a convenience of the low-stand period, the construction period is short, the accuracy is high and the structure is stable.