In this paper, the stress and stability of the joint blocks between RCC cofferdam in the third stage construction of the Three Gorges Project(TGP) and longitudinal concrete cofferdam are computed using finife element method. The major calculations include the deformation and stress fields of joint blocks under the action of main loads such as water pressure and uplift pressure. The effects of joint grouting among blocks and water seal are studied respectively. Based on above computation, overall and local stability of resistance to sliding on the foundation surface are analysed. The calculation results show that the effects of uplift pressure and water seal on joint blocks are obvious, and the joint blocks possess enough safety degree of resistance to sliding.