The present invention belongs to the technical field of disaster prevention and reduction of transmission lines, and provides a quasi-static calculation method for lateral unbalanced force of transmission lines. The present invention can quantify significant unbalanced force caused by a failed tower to an adjacent tower, thus to quantitatively evaluate cascading failure risk of transmission lines. The present invention does not need a lot of iteration, and can parametrically reveal influence of various factors on unbalanced force. The present invention quantifies influence of large deformation effect of a conducting wire on wind load, and at the same time, wind load of the conducting wire is corrected by the trapezoidal equivalent wind pressure proposed, which avoids wind load calculation error of the conducting wire caused by a relatively large vertical height difference, better reflects actual mechanical state of the lines, and optimizes existing wind load calculation method of transmission lines.