Abstract: To address the challenges of ultra-wide cofferdam design, construction stability control, excavation organisation for irregular-shaped foundations, and crack prevention and waterproofing for large-volume concrete in ultrawide cofferdam-built tunnels under complex marine conditions, this study summarises and proposes systematic solutions based on the cofferdam-built tunnel section of the Shenzhen-Zhongshan Channel East Artificial Island project. Field application indicates that the design approach employing double-row sheet pile cofferdams and sub-cofferdams effectively ensures cofferdam stability, with structural deformation being virtually negligible. Employing differential global positioning system (DGPS) integrated with HYPACK software for dredging operations, supplemented by extending sheet pile lengths and implementing jet-grouting reinforcement in geologically vulnerable zones, effectively prevents sheet pile leakage and minimises cofferdam deformation; Excavation of irregular deep excavations adheres to the principle of "overall zoning and sequential construction by sections" Individual excavation pits follow the construction principle of "overall retaining, dewatering assistance, vertical layering, longitudinal sectioning, shoring before excavation, and sequential construction of the structure from bottom to top" effectively ensuring construction organisation. Employing the optimal concrete mix design for the main structure of the cut-and-cover tunnel, constructed using the seawall construction method, combined with the independently developed intelligent temperature control and circulating cooling water system for marine bulk concrete, can effectively reduce the occurrence of cracks and leakage in large-volume concrete.