The invention provides an underwater grouting template structure with an increased cylinder section and a construction method thereof. The underwater grouting template structure comprises a template undersurface foundation, a single-section steel template, an idler wheel slideway structure, fine-adjusting and locating structures, tapered guide structures, and a bottom grout stop rubber strip. The single-section steel template is arranged around an existing cylinder, and is formed by splicing two half single-section steel templates so as to realize underwater segmentation sinking assembly; the idler wheel slideway structure is arranged on the existing cylinder so as to be used for realizing guiding, supporting and limiting of underwater sinking of the single-section steel template; the fine-adjusting and locating structures are arranged on the periphery and the inner part of the single-section steel template; the tapered guide structures are located on a top-face flange plate and a bottom-face flange plate of the single-section steel template; and the bottom grout stop rubber strip is located on the bottom face of the single-section steel template at the bottom section. The underwater grouting template structure with the increased cylinder section provided by the invention can meet the requirement of application environment of the increased underwater cylinder section of a cover structure such as a bridge floor or a wharf surface on the top part of the cylinder, so that the underwater grouting template structure difficult to construct and the construction method thereof are easy to operate, the automation degree is high, the quality is reliable, the construction risk and the cost are reduced, and the work efficiency is improved.