In order to accurately analyze the condition of stress ribbon in the stress ribbon bridge, a method for calculating the stress ribbon supported by saddle is proposed. First, the bending moment equilibrium differential equation of stress ribbon under uniformly distributed load is established. Then, the undetermined coefficient of the general solution of the differential equation is determined by the boundary conditions that the deflection of the stress ribbon is about zero and the radius of curvature is the radius of the saddle at separation point of saddle. Finally, the supporting length of the saddle is calculated backwards according to the geometric coordination relationship and solved by iteration. To verify the correctness and practicality of the method, the result of this method is compared with the results of the finite element simulation calculation and the hinged boundary calculation formula. The result shows the calculating formula of this method is correct, can efficiently find the separation point of stress ribbon on the saddle, and is more accurate than the hinged boundary calculation formula, which is more suitable for application in practical projects.