Blasting demolition of cofferdam is usually carried out by stages and areas to ensure the safety of hydraulic structures near the blasting zone and to reduce the difficulty of blasting and the workload. To secure the global stability of cofferdam slope during phased demolition, and to prove the feasibility of the phased demolition scheme, we analyzed the stabilities of intake cofferdam in three cases: after being thinned and before borehole drilling damage, during thinning process under blasting loads, and after borehole drilling damage. The phased demolition project for the intake cofferdam of extension project of north bank of Kariba hydropower station in Zambia is taken as a case study. Three-dimensional finite-difference method in association with strength reduction method and pseudo-static method were adopted. The results of numerical simulation showed that the safety factor of cofferdam slope was 1.43 after being thinned and before borehole drilling damage, 1.39 during thinning process under blasting loads, and 1.32 after borehole drilling damage, all met the requirements of standard. These results reveal that the effects of thinning process on the cofferdam and the cofferdam slope on land side are only local, with no serious impact, indicating that the phased demolition scheme is safe and rational.