Collapse accidents in narrow braced excavations caused by over-excavation occur frequently. However, the mechanism of progressive collapse and corresponding control measures remain underexplored. Based on the excavation collapse accident of Xianghu Station of Hangzhou Metro Line 1 in 2008, the numerical simulation method was adopted to study the mechanism of its initial failure and subsequent collapse. Control measures to prevent continuous collapse were proposed, and the reliability of the original excavation design and the improved excavation design under different over-digging depths was analyzed by using the excavation reliability calculation method. The results show that over-excavation can lead to tensile failure of the first and the second layer of struts, reducing the redundancy of the retaining structure and triggering initial failures, which result in larger-scale collapses. Replacing the first layer of strut with a reinforced concrete strut effectively enhances the redundancy of the retaining system, reducing the risk of progressive failure and the scope of collapse. However, when the strut initially fails, the concrete strut at the first level does not effectively improve the reliability of preventing subsequent failure to the excavation. Instead, the overall reliability of the excavation should be enhanced by maximizing the strength and stability of the steel struts.