Abstract: For large-span tunnels such as double-arch tunnels, the stability of the mid-wall is critical for stabilizing the surrounding rock, and the thickness of the mid-wall is regarded as the key to safe and economical construction. Additionally, construction safety, cost, and progress are directly affected by the construction procedures. In this paper, a random reliability-based analysis is carried out regarding the risks in double-arch tunnels, and the minimum thickness of mid-wall is optimized and compared with the requirements given in the specification.