The utility model discloses a tension detecting device for a steel cable. The utility model is a symmetric mutually-hinged four rods frame structure, wherein, each of the two end bars is a cable fixture (4); the two side bars are formed through the concatenation of a tension sensor (5) and a hydraulic cylinder (2) respectively. The utility model can be used to detect the practical bearing tension of a steel cable in a cable-stayed bridge, a suspension bridge, a tied arch, and other structure using a cable to bear weight, and provide technical means for ensuring engineering quality. The utility model needs no any assumption and conversion and can detect the bearing tension of the steel cable directly.