Abstract: Aimed at the challenge of large sidewall deformation encountered during the construction of a water conveyance tunnel for the Qingshuihai Water Purification and Distribution Project, located in Class V strata of siltstone interbedded with mudstone, this paper investigates and proposes effective control technologies. The deformation mechanism of the city-gate-shaped tunnel was revealed by capturing deformation patterns through field monitoring combined with theoretical analysis. The effectiveness of a composite support scheme, consisting of "pre-support + systematic rock bolts + steel arches + shotcrete + bottom steel struts, " was comparatively analyzed using numerical simulation methods. The study indicates that the large tunnel deformation results from the coupled effects of multiple factors, including high in-situ stress, the low strength of the weak rock mass, tectonic stress, and groundwater-induced softening. The proposed composite support technology effectively controls the deformation magnitude and confines the plastic zone development within a controllable range. Field application has verified that this technology reduced the maximum convergence from 318 mm to 84 mm and the peak rate from 140 mm/d to below 25 mm/d, thus resolving the large deformation problem. The findings can provide technical support and a theoretical basis for similar engineering projects.