The invention provides a steel pile casing settling and positioning method applying a steel pile casing cofferdam. The method comprises the following steps: 1, constructing drilling platform steel pipe piles, and constructing drilling platform supported trestles on the drilling platform steel pipe piles; 2, machining the steel pile casing cofferdam; 3, delivering the steel pile casing cofferdam to the settling position; 4, sliding the drilling platform steel pipe piles downwards along the steel pile casing cofferdam through a floating crane till the seabed surface is reached; 5, mounting a pile casing positioning rack on a drilling platform, hoisting steel pile casings with the floating crane, and vertically descending the steel pile casings to a steel pile casing descending bin along the positioning rack; 6, conducting pouring on the interior of the steel pile casing descending bin. According to the steel pile casing cofferdam and the steel pile casing settling and positioning method applying the same, the cofferdam and the drilling platform steel pipe piles are connected through connecting and extending-and-contracting devices, the cofferdam is positioned and descended through the drilling platform steel pipe piles, then the cofferdam is descended to the seabed surface along the drilling platform steel pipe piles, interference with descending of the cofferdam caused by wave current force is avoided, and cofferdam descending precision is guaranteed.