The outer containment dome of the ACP1000 nuclear power plant adopts large-span steel formwork as the supporting bottom formwork for concrete pouring, with a span loss ratio of about 1/4, and has the characteristics of thin thickness and large span. The upper concrete pouring load is large, and according to the specifications, its stability should be analyzed. This article focuses on the one-time pouring of concrete with steel formwork. Based on the entire elastic and elastoplastic processes, detailed analysis is conducted comparing the consideration of defects and perfects. Combined with empirical formulas, a multi-stage pouring scheme and calculation method for steel formwork are proposed. Furthermore, stability analysis methods, results, and conclusions are provided for multi-stage pouring, ensuring the safety of stability bearing capacity and providing reference value for the stability analysis of steel formwork for nuclear power plant domes in the future.