Abstract: Introduction The assumption of rigid floor slab is a simplified analysis method which is widely used in civil building structure and has strong applicability. In general, the interlayer and opening of the civil structure are less, but the interlayer and opening of the power plant structure are generally more. This paper aims to study the influence of rigid floor assumption on calculation results with large openings. Method In this paper, a large general finite element software MIDAS was used to establish a hypothetical ideal opening structure model, typical slab large open hole civil architecture model, and a large nuclear power plant conventional island workshop structure model. Result The results show that the assumption of rigid floor would cause a certain degree of distortion to the calculation results of such structures. Conclusion Therefore, it is necessary to pay attention to the effect of the large opening structure, especially in the analysis and application of the power plant structure, and not to use the rigid floor assumption as long as possible.