The invention relates to the technical field of civil construction engineering, in particular to a tie piece for a sandwich composite outer wall board and a construction method thereof. The tie piece is characterized by being provided with a long-strip-shaped main body; circular holes for penetrating of additional steel bars are formed in the two ends of the long-strip-shaped main body correspondingly; barb-shaped connectors are arranged at the two ends of the long-strip-shaped main body correspondingly, and the barb-shaped connectors and the main body are of an integrated structure; the barb-shaped connectors comprise barb-shaped extending parts symmetrically arranged on the left side edge and the right side edge of the main body front end and the left side edge and the right edge of the main body rear end; and the barb-shaped extending parts can be parallelogram extending parts each with two opposite acute angles, wherein the long edges are connected with the main body, and one of the opposite acute angles of each parallelogram extending part form a barb facing the middle of the main body. Compared with the prior art, the tie piece has the significant advantages of being high in performance, multifunctional, high in durability, environmentally friendly, capable of saving energy and healthy in the using period, and suitable for both reinforced concrete structure engineering and steel structure engineering.