During urban renewal in built-up areas, the conflict between safety control and cost optimization for deep and large foundation pits is increasingly pronounced. The combined support system integrated with passive zone reinforcement represents an effective approach to resolving this technical challenge. Drawing upon a foundation pit project for the upgrading and retrofitting of an industrial plant in Chengdu, this study addresses complex boundary conditions including deep and large excavation dimensions, thick soft soil strata, uninterrupted plant operations during construction, and close proximity to existing structures. A combined support system was proposed and implemented in a zoned and segmented manner, comprising quincunx-pattern double-row piles, graded slope excavation with shotcrete-anchorage, steel struts, and top-down inter-pile lagging panels, all integrated with passive zone lime pile reinforcement. Field monitoring results demonstrate that all deformation indices of the foundation pit remained within code-specified limits, thereby ensuring the safety of the excavation and the surrounding environment under uninterrupted production conditions. Moreover, differentiated measures, such as utilizing soldier piles to serve dual function as permanent structural components and applying high-pressure jet grouting piles for reinforcement in sensitive zones, effectively shortened the construction period and reduced overall costs, yielding notable economic and social benefits. This foundation pit project validates the rationality and reliability of the combined support system integrated with passive zone reinforcement, and offers a referable technical pathway and practical experience for similar deep and large foundation pit projects in the Chengdu Plain.