The invention provides a beam transporting method for building of double-deck bridges. The beam transporting method includes the steps of firstly, supporting one end of a main beam (3) on an upper deck (2) and supporting the other end of the main beam (3) on a bridge pier (4); secondly, transporting a lower-deck U-beam (8) to the position of a tip (9) of the lower deck through a beam transporting vehicle (7); thirdly, lifting the lower-deck U-beam (8) to move to a lower-deck falling position (10); fourthly, paving a lower-deck rail (6) on the lower-deck U-beam (8), transporting an upper-deck U-beam (11) to the lower-deck rail (6) through a beam transporting vehicle (5), and lifting the upper-deck U-beam (11) and moving it to an upper-deck falling position (12). By the beam transporting method for building of double-deck bridges, the lower-beck U-beam needed to be transported can be transported to the position right above the lower-deck falling position conveniently rapidly, and the upper-deck U-beam for the upper deck can be transported by the aid of the lower-deck rail conveniently rapidly.