The invention discloses a steel sheet pile support construction method for building an underground garage open caisson to reduce the construction complexity and improve the construction efficiency during open caisson building and reduce the building expenditure for the open caisson. According to the construction method, open caisson construction is carried out in an area defined by a steel sheet wall, wherein building of a wall of the open caisson is carried out in a manner of segment-by-segment construction from bottom to top. The building method comprises the steps that steel sheet piles are sunk, and the continuous steel sheet pile wall is formed; a certain of earthwork is excavated in the area defined by the steel sheet pile wall, then, a pile block enclosure purlin is erected on the top of the steel sheet pile wall, and horizontal braces are built on the inner side of the pile block enclosure purlin; and then, open caisson building is finished through a method that earthwork is excavated segment by segment, the wall is poured segment by segment and sinking is carried out.