Take a real 500 kV straight tower as an example, the safety status of the tower is evaluated based on the insulator string tension measurement system of the ice-coated monitoring terminal. This paper proposes a method for calculating the insulator string tension considering the coupling of both spans under non-uniform ice-coated condition, and obtains insulator string tension and inclination angle under various icing conditions. By finite element simulation, the error of the proposed method is less than 1.98%. The finite element model of the straight tower is established, and the critical failure conditions under different gravity loads and unbalanced forces along the line are analyzed, and the grading critical failure curve is drawn up. The ice thickness range of the straight tower critical failure under none-uniform icing is analyzed. This paper combines the engineering practice and brings guidance for failure pre-warning of straight towers in icing areas.