应用传统的极限分析理论，对可简化为刚塑性的材料本构关系提出了一种改进的“刚性－非线性强化”近似模型。它较现有的几种刚塑性简化模型更符合实际，因而更合理。在梁的极限分析中采用这一新的刚塑性模型，并推导出了极限弯矩公式。文中以矩形截面梁为例，对所得结果与传统公式作了比较，表明后者可视为本文结果的一个特例。本文最后还给出了便于工作应用的简化公式。文中所提出的刚塑性简化模型也可用于刚架、薄板等结构的极限分析。 By applying the traditional limit analysis theory, an improved “rigid - nonlinear hardening” approximation model is proposed for the constitutive relation of material which can be simplified to rigid plasticity. It is more realistic than some of the existing rigid-plastic simplified models and is therefore more reasonable. The new rigid-plastic model was adopted in the limit analysis of beams and the formula of ultimate moment was deduced. In this paper, a rectangular section beam is taken as an example to compare the result with the traditional one, which shows that the latter can be regarded as a special case of the result in this paper. Finally, this article also gives a simplified formula for work applications. The rigid-plastic simplified model proposed in this paper can also be used for the ultimate analysis of rigid frame, thin plate and other structures.