The shield launching shaft of the Yellow River crossing tunnel on the central route of southtonorth water transfer project has the thickest and deepest diaphragm walls in China and is the deepest foundation pit in China. As a typical extradeep foundation pit works, the launching shaft, due to the complex geology conditions, complex loading conditions and complex construction conditions, has great construction difficulties and great risks. Construction technologies, including topdown construction of diaphragm walls, monitoring and information feedback, are adopted on basis of the actual conditions, which successfully solves the structural stability problems caused by shaft sinking and supporting and successfully avoids all the construction risks caused by traditional foundation pit supporting technologies. Topdown construction techniques, key construction processes and key control points are presented in the paper on basis of the construction practice. Furthermore, problems occurring during construction are analyzed and countermeasures are proposed.