The invention discloses a construction method for supporting enclosing purlins of foundation pits in water area, which relates to a supporting structure for building the foundation pits. The method mainly comprises plane arrangement of foundation pit construction in water area, concrete girt and supporting construction, and a steel supporting construction process. Finished partial hoist runway pile foundations are utilized to serve as the basis, a foundation pit is connected with land area by adopting a method of building a temporary trestle according to the local water level and the service conditions of the trestle, a combination mode that a tower crane is arranged at the upper part of the foundation pit and a crane ship is arranged outside the foundation pit, is used to hoist a large quantity of materials, a moving platform is placed on a first concrete brace by channel steel, a wood board is paved on the channel steel, an operating platform is used for segmented pavement, and is moved to the next section for repeated use when the construction at one section is finished, thereby finishing the construction method for the supporting enclosing purlin of the foundation pit in waterarea through the steps of concrete girt and concrete supporting construction and steel supporting construction.