In order to obtain the flexural strength of external prestressed concrete beams, the ultimate stress of external prestressed tendons must be determined first. In the past half century, many scholars have conducted experiments and analysis on the ultimate stress of unbonded prestressed tendons in vitro or in vivo, but these researches mainly focus on steel and rarely on fiber composite materials (FRP). Based on the relevant experimental data, the difference and change of the ratio between the length of equivalent plastic zone and the neutral axis of the failure section are analyzed in the case of FRP and steel reinforcement. The analysis shows that for external prestressed concrete beams with FRP, this ratio is stable and can be a constant. However, this constant is different from that of the components with steel as external prestressing tendons. In this paper, the conversion relation between the length of equivalent plastic zone and the ratio of neutral axis of failure section is established when FRP and steel are prestressed in vitro respectively. On this basis, it is suggested that it can be applied to both simple supported beams and continuous beams. The calculation method of the ultimate stress of external prestressed FRP bars is suitable for both conventional external prestressed steel bars and non-conventional external prestressed FRP bars.