Based on the discussion of the simulation for the cable prestress, the rigidity theory was used to identify the geometrical dimensions of semi-rigid structures in zero state. When the structural geometry in initial state is given, the geometrical dimension in zero state is determined by the initial prestress distribution. The contribution-matrix method is firstly applied to get the initial lack of fit. However, the influencing factor of initial lack of fit on the displacement of control point is linear superposition, that is, the structural response is linear, and it is irreconcilable with the geometry nonlinear of semi-rigid structure. In order to reduce the error of nonlinear structural response, the influencing factor in the contribution-matrix method is revised, and modified contribution-matrix method is put forward. Programs by ANSYS parameter design language APDL are worked out for the form-finding analysis of beam-string structure of a practical project. The numerical results show that the modified contribution-matrix method reduces the errors of the assumption of structural linear response. And it can be applied to the form-finding of semi-rigid structures with small stiffness.