In this paper, representative methods for calculating the ultimate stress of extracorporeal prestressing tendons at home and abroad are firstly reviewed. On this basis, a formula for calculating the ultimate stress of extracorporeal prestressing tendons in the design of new and strengthened bridges is proposed. The formula is simple, intuitive, and applicable to both simply supported and continuous beams. Since the modulus of elasticity of prestressing tendons is included in the formula, the proposed formula can also be used for the extracorporeal prestressing structures made of non-steel materials.