Abstract: The transition from small section tunnel to large section tunnel is one of the difficulties in tunnel con? struction. Based on the Xinlian running tunnel section of Shenzhen metro lot 2131, this paper studies the construction scheme optimization for transformation from G5 small-section tunnel to C5 large-section tunnel, uses three-dimensional finite element method to calculate and analyze the advantages and disadvantages of various construction methods, and compares the calculation results with monitoring data. The results show that using the construction method of longitudinal climbing and roof-ripping, it can better control the deformation of surrounding rock and ground settlement after tunnel construction, avoid the initial support deformation and cracking caused by the longitudinal extrusion of fractured surrounding rocks as far as possible and reduce the construction risk.