To conveniently calculate the tension force of each suspension cable of self-anchored suspension bridge during system transformation in design phase, a practical calculation method was proposed based on the internal force state of main cable. The main cable was divided into tension section and free suspension section according to the tension degree of the suspension cable. Based on the idea of displacement method in structural mechanics, the internal force of main cable tension section was made consistent with that of target state by adding restraints at the corresponding suspension points of main cable. Then the free deformation of each main cable segment was balanced by releasing the restraints to obtain the main cable state under this condition. Considering the energy conservation of each main cable segment before and after restraint release as well as the deformation compatibility conditions of the main cable, the equilibrium equation of the main cable was established and solved to obtain the internal forces of each main cable segment after system transformation. Next, according to the vertical unbalanced force between the main cable segments of the self-anchored suspension bridge, the cable forces under this condition were calculated. By comparing with the test results in engineering examples, it shows that the proposed method is convenient and concise, which is independent of the effect state of the former stage. The calculation results of the proposed method met the accuracy requirements, which can be applied in the system transformation analysis and structural design analysis of general self-anchored suspension bridges and might be the guidance for structural design and optimization.