The invention relates to an ultra-long cantilever structure formwork supporting steel truss. A plurality of truss bodies are parallelly stretch out of a floor layer N of a required cantilever position; each truss body is provided with two parallel horizontal rod pieces (1) and two shear force supports (2) between the two corresponding horizontal rod pieces (1); each horizontal rod piece (1) on the upper portion is sequentially provided with five connecting joints of J, K, L, M and N from outside to inside; and each horizontal rod piece (1) on the lower portion is sequentially provided with four connecting joints of D, E, F, and G from outside to inside. The N connection joints are arranged on the end face of an upper floor, and the G connecting joints are arranged inside the edge of a lower floor. The K connecting joints of the horizontal rod pieces (1) on the upper portions correspond to the D connecting joints of the horizontal rod pieces (1) on the lower portions in position. The shear force supports (2) are arranged on the line segments corresponding to the DJ, DK, DL, EL, EM, FM, FN and GM connecting joints. According to the method, construction is programmed, the construction quality is guaranteed, the construction safety is improved, and the ultra-long cantilever structure formwork supporting steel truss and the method are suitable for widespread popularization.