The invention relates to the technical field of beam string structure construction, in particular to a construction method for a U-shaped beam string structure. The construction method comprises the following steps: A, installing a pipe truss of one end, and then, performing section construction of an upper string box beam, wherein a construction sequence is that construction is performed from one end to the other end; firstly, installing the upper string box beam of one end, inserting roof secondary beams into the upper string box beam, and then, installing a pipe truss, an upper string box beam and roof secondary beams of the other end; B, installing struts; and C, installing a lower string stay cable, wherein the stay cable comprises a main cable and knee-bracing tension rods; in a tensioning process of the lower string stay cable, each upper string box beam may produce an upward displacement, and automatically separates from a temporary support frame to realize automatic unloading. According to the construction method, a tensioned unit is enabled to be located below lateral restraints of structures of two sides in a structural plane, and therefore, plane stability of the beam string structure is guaranteed during tensioning construction, and structural stiffness of the beam string structure at the construction stage is guaranteed.