Double-shield tunnel boring machine (TBM) construction is an important mechanized excavation method for long hydraulic tunnels. However, in mountainous water-conservancy projects, portal construction sites are often confined and tunnel alignments commonly include multiple curves. Conventional out-of-tunnel integral assembly and straight-line launching are therefore difficult to implement. Taking the headrace tunnel of the Meishan Irrigation District Project in Hunan Province as a case study, this paper proposes an assembly and construction method characterized by “out-of-tunnel assembly, split-body stepwise advancement, and integral launching”, based on the portal site conditions and TBM dimensions. For the limiting launching condition with a curve radius of 500 m, a series of technical measures are adopted, including optimization of the launching-tunnel section, pre-deflection of the curved guide bench, directional arrangement of the reaction frame, precise control of the first segment ring, and micro-correction with low thrust and short advance strokes. The results show that the proposed technology effectively solves the problem that an approximately 200 m long double-shield TBM cannot be integrally assembled within a 40 m × 40 m confined site. It enables orderly installation and smooth launching of the main machine, connecting bridge, backup gantries, and belt conveyor system. During launching in the limiting 500 m radius curved section, the TBM attitude remains generally controllable, with no obvious shield interference or belt conveyor rollover. In the trial-excavation stage in sandy slate strata, all tunnelling parameters are below the design limits of the machine, and the equipment operates stably. The monthly advance in March increases to approximately 444 m, and the maximum daily advance reaches 23.39 m. The research findings provide scientific and reliable theoretical and practical support for split assembly of double-shield TBMs and launch operations with a 500 m Limiting-radius Curve.