The study in the paper starts with beam distortions , and it treats the plastic deformation area as pure bending sect , rest of the beam as rigid bodies . according to different shapes of the external tendons , it divides into three structural forms familiar in projects , that is the beams with no deviator , with a deviator and with two deviators , and studies the relation between the deformation of the external prestressing tendons and that of the beams , and derives the formulas of the ultimate stress increment of external tendons . in the formulas , the ultimate angle corresponding to half of the plastic deformation area is considered as a variable parameter 本文首先从梁极限状态下的变形入手,将塑性铰区看作纯弯段,将塑性铰以外的区域看作刚体,按照体外预应力筋的形状不同,分工程中最常见的三种体外结构形式,即无转向块的直线型体外筋混凝土梁、具有一个转向块的单折线型梁和具有两个转向块的双折线型梁,研究体外预应力筋变形和梁体的变形间的关系,推导出以极限状态下塑性铰区之半对应的转角为参数的计算体外预应力混凝土简支梁以及连续梁的体外筋应力增量计算公式,并进一步求得梁的极限承载力。