The invention relates to a combined semi-trailer body which comprises a semi-trailer plate and a plate type supporting component, wherein the plate type supporting component is positioned just below the semi-trailer plate, and the semi-trailer plate and the supporting component are respectively formed by more than two fiber reinforced composite plates which are spliced through a splicing tank and bonded with resin. The invention further relates to a manufacturing method of the semi-trailer body. The combined semi-trailer body disclosed by the invention can be formed by a plurality of lightweight fiber reinforced composite plates which are spliced and combined, and the structure is simple. The manufacturing method disclosed by the invention has the advantages of simple process, high production efficiency, high component strength, difficulty in stripping because double effects of binding and mechanical connection are adopted, and obvious strong quantitative effect because the semi-trailer body is made of pure composite materials, and the manufacturing method is suitable for lightweight transformation of various vehicle types.