The mechanical behavior of composite C-beam webs under flexure-shear coupled loading was investigated through a combination of experimental testing and finite element analysis. A series of experiments were conducted to evaluate the buckling and post-buckling characteristics of both the opening and reinforced specimens. Test results revealed that the original web experienced global buckling at 40 kN and ultimate failure at 70 kN, showcasing significant post-buckling capacity. Following reinforcement of the critical region with an H-shaped frame, a marked improvement in load-carrying capacity was observed. During loading, the reinforced specimen exhibited no signs of global buckling before reaching its ultimate failure load of 190 kN. Notably, this represented a 2.7-fold increase in bearing capacity compared to the opening configuration. To further understand the underlying mechanics, three-dimensional finite element models were developed for intact, opening, and reinforced specimens. The Hashin damage criterion was implemented to simulate the progression of material degradation. The FEA results showed good correlation with experimental observations in terms of buckling load, failure mode, and ultimate strength. Additionally, the numerical simulations effectively captured the damage propagation mechanisms within the composite web structure. This study offers a robust computational framework for optimizing the design and retrofitting strategies of composite beam webs.