To support the high-power and long-pulse operation goals of the experimental advanced superconducting Tokamak (EAST), an upgrade was carried out on the ion source of the neutral beam injection (NBI) system, increasing the beam power from 4 MW to 6 MW. This power enhancement exposes the back plate, a key component, to extreme thermal loads, with a surface heat flux reaching up to 13.18 MW/m². Accordingly, an evaluation was conducted on the cooling performance of this component, focusing on the effects of structural optimization on its temperature field distribution and thermal stress characteristics. The results show that the peak surface temperature of the back plate is reduced from 409.31 ℃-far exceeding the softening threshold of oxygen-free copper (300 ℃) to 298.71 ℃, well below the softening threshold of chromium-zirconium copper (550 ℃), thus avoiding overheating, softening, and even failure of the material. The optimized peak thermal stress is 281.21 MPa, lower than the tensile strength of chromium-zirconium copper (520 MPa), and the thermal deformation elongation is 0.433% (≪3% threshold), which guarantees mechanical safety and geometric stability. This work provides critical technical support for the long-term stable operation of the ion source at a beam power of 6 MW.