In view of the complex structure of the thick silt foundation and the problem of the cofferdam stability, the sloping stability of high earth-rock cofferdam on thick silt foundation under typical conditions was computed by limit equilibrium method, and the sensitivity analysis of slope stability against sliding was also analyzed according to the possible changes of physical and mechanical parameters of cover layer and lens body in the process of design and construction. The results showed that there were situations which can not meet the sloping stability requirements of cofferdam and foundation in typical conditions; the changing of internal friction angle in the second covering layer had an obvious influence on the sloping stability of cofferdam; the lens parameters had significant impact on the situation of the most dangerous sliding surface passing through the lens, so it should consider the foundation reinforcement measures.