The sandbag cofferdam of 20 m height and 800 000 m3 volume and plastic concrete antiseepage wall are built in order to realize connection between underwater immersed tunnel and shore ramp. The construction technology of largescale sandbag cofferdam used in Honggu Tunnel in China and the design adjustment of plastic concrete antiseepage wall are introduced. The clay antiseepage wall is replaced by plastic concrete antiseepage wall which can guarantee the safety of Old Town of Nanchang. The sandy cobble pad and gabion mat are replaced by polypropylene woven fabric + small sandbag for protecting cofferdam slope. The construction practice shows that the sandbag cofferdam is a new, advanced and feasible retaining structure in inland river and it is worth popularizing.