The discharge capacity of deep drainage holes in high-head hydraulic structures is closely correlated with the structure safety. The turbulence characteristics of the flow in front of the deep hole with a fully opened radial gate (1.20 m) was simulated by RNG k-ε turbulence model. The results indicate that as the applied water head increases, the velocity gradient of the near-wall flow also increases, leading to a corresponding rise in the thickness of the turbulent boundary layer. Consequently, the mean turbulent shear stress, flow turbulence and energy dissipation increases Significantly. As the applied water head increases from 12.5 to 52.5 m, the maximum thickness of the turbulent boundary layer, maximum shear stress, maximum turbulence intensity and maximum turbulence dissipation rate in front of the gate increases by 0.67, 4.57, 0.89, and 5.50 times, respectively. Under these combined effects, the discharge coefficient of the discharge flow first increases and then decreases with the rising water head (increases from 0.915 to 0.931 and then decreases to 0.930). Notably, at an applied water head of 42.5 m, the discharge coefficient, which characterizes the drainage capacity of the gate opening, reaches its maximum value. With continued increase in the applied head, although the discharge flow continues to rise, the increment progressively diminishes, indicating a corresponding decline in the gate’s surge discharge capacity. It indicates that with the increase of applied water head, the thickness of the turbulent boundary layer in deep holes grows (the effective flow area decreases), the turbulence intensity increases, and the energy dissipation of the flow in front of the gate rises, leading to a reduced increase in the flow coefficient, or even negative growth. This should be considered in the calculation of deep hole discharge capacity.