The invention discloses a dual-purpose device serving as both a pier column formwork and a prestress support and a construction method of the dual-purpose device. When construction is conducted, a pier column lower portion formwork is assembled; then the dual-purpose device is installed on the upper portion of the formwork, and the pier column lower portion formwork and a steel formwork of the dual-purpose device serve as a pier column pouring formwork; after installation is accomplished, pier column concrete is poured, and after the strength of a pier column is equal to the form removal strength, only the portion, located at the bottoms of hoops, of the formwork is disassembled; assembly and erection of a bailey beam are conducted, a distribution beam and a full framing support on the topare erected, a bottom form is laid, and side forms are installed; then finish-rolled screw-thread steel is subjected to prestressed tension; a beam body of a capping beam is constructed; after maintenance is accomplished, a disc buckle support and bailey pieces are disassembled in sequence; and finally, the finish-rolled screw-thread steel is tensioned, and the dual-purpose device serving as botha capping beam formwork and the prestress support is disassembled. The dual-purpose device serves as the formwork for pier column pouring, and also serves as a supporting device for the upper cappingbeam pouring process, the construction procedure is simplified, the construction period is effectively shortened, foundation treatment is not required, and the material and the construction space aresaved.