Wall-detached flow inside an ultra-supercritical steam turbine control valve was comprehensively investigated with detached-eddy simulation, proper orthogonal decomposition (POD), and flow reconstruction. The dependency of the wall-detached flow on the control valve’s opening ratio and pressure ratio was established first. Scattered wall-detached-flow, merged wall-detached-flow, and intersected wall-detached-flow were then identified by distinguishing the detachment scale of the wall-detached jet. Subsequently, flow analysis was conducted in terms of the statistical flow quantities, i.e., velocity fluctuation, turbulent kinetic energy, pressure loss, and pressure fluctuation. The statistical results demonstrated that the merged wall-detached-flow facilitated the most intensive velocity and pressure fluctuations inside the steam turbine control valve. The intersected wall-detached-flow encountered significant shock-wave reflections along the downstream pipe. By conducting POD analysis and flow reconstruction on the instantaneous flow snapshots, the dominant vortex structures and energetic pressure fluctuation modes were extracted to illustrate the wall-detached flow’s unsteady behavior. The results showed that the instabilities of the scattered wall-detached-flow were primarily represented by the horizontal flapping motion of the wall-detached jet. However, for the merged wall-detached-flow, both the vertical out-phase oscillation and the horizontal flapping motion of the wall-detached jet intensified, yielding essential axial pressure fluctuation modes. As for the intersected wall-detached-flow, due to the complex wave reflections and propagations, essential regions with velocity discontinuities and diagonal crosslines with intensive pressure fluctuations formed inside the valve pipe. These findings are of great practical significance for the operation and optimization of steam turbine control valves in thermal power plants.