Abstract: The cofferdam cut-and-cover method is a common underwater tunnel construction method. The seepage control and the stability of the cofferdam are critical to the safety of the entire tunnel construction. The deep embedded and thick cobble and peaty soils which are widely deposited underneath the project site impose a significant challenge for the stability and seepage control of the cofferdam. To overcome these difficulties, the riprap plus Trench cutting Re-mixing Deep Wall (TRD) method was used in the excavation of the tunnel underneath the Qingshan Lake project. The result shows that the 30 day cumulative surface settlement of the earth-rock cofferdam structure combined with riprap and TRD was less than 4.5 mm, the maximum horizontal displacement change was less than 7 mm, and the average permeability coefficient of cement and soil was 4.30×10^-7 cm·s^-1. The stability and the seepage resistance of the cofferdam could meet the design needs.