Abstract: The determination of the effective length of the tower column in a cable-stayed bridge is a key problem in estimating the ultimate state of its load-carrying capacity. Based on the mechanical characteristics of a stay cable supporting deck elastically, a new method and a mechanical model, and a transfer matrix method and a triple-beam mechanical model with discrete springs, are proposed for calculating the effective length of the tower column of a cable-stayed bridge with a rigid frame system. In the model, the tower column and main girder are regarded as Bernoulli-Euler beams with the consideration of axial deformation, and cables are simplified as springs with mass. By utilizing the transfer matrix method, the stability characteristic equation of the system is derived considering the boundary and continuity conditions. In this way, the effective length factor for the in-plane stability of the tower is obtained. Finally, the case study and its comparison with results obtained by finite element method verify the validity of the proposed theory and processing method, which can meet the needs of the computational analysis of a single-tower cable-stayed bridge with a rigid frame system and, of the evaluation of effective length of the tower column.