Water inrush and bottom floating must be avoided in the construction of ultradeep foundation pits at riverside under high confined water. Therefore, rational construction procedure should be planned for such foundation pits so as to ensure the stability of the retaining structures and the foundation pits themselves and to provide safe conditions for the construction of the main structures in the foundation pits. In the paper, the construction of the foundation pit of Meizizhou ventilation shaft of Weisanlu rivercrossing tunnel in Nanjing is studied. The challenges in the construction of the foundation pit are analyzed, technical countermeasures are taken in the aspects of retaining structure, excavation method, implementation plan and construction procedure, the technical scheme of underwater excavation and underwater bottomsealing is adopted, and the optimum construction procedure is determined according to the calculation analysis results. Conclusions drawn are as follows: 1) For ultradeep foundation pits where the excavation depth and the confined aquifer thickness are so large that it is difficult for the waterproof curtain to penetrate the confined aquifer, the application of underwater excavation can effectively avoid water inrush and can improve the force and deformation state of the retaining structures; 2) The underwater bottomsealing concrete, which can bear the high pressure of the confined water, is the key to the successful implementation of the project.