Perovskite polycrystalline films have gained considerable attention as promising materials for X-ray imaging applications. However, the conventional fabrication of perovskite-based X-ray detectors typically relies on large quantities of highly toxic solvents, especially since the absorber layer must reach thicknesses of hundreds of micrometers to ensure sufficient X-ray attenuation. To address this issue, we employed green and non-toxic γ-valerolactone (GVL) as the solvent for fabricating MAPbI₃ perovskite thick films. Accompanying this, an ultrasonic-assisted annealing process was introduced to mitigate the irregular migration and inhomogeneous distribution of solute ions resulting from the weak coordination between GVL and Pb²⁺. This combined strategy enabled the production of MAPbI₃ thick films with remarkable uniformity in both horizontal and vertical directions. The inhomogeneity values below 38.19% along the vertical axis and less than 6.6% in the horizontal plane were achieved, exhibiting an 84% improvement in dark current uniformity over films prepared by conventional annealing methods. Furthermore, the resulting MAPbI₃ thick-film detector exhibits a high X-ray detection sensitivity and a high imaging resolution of 4.46 lp·mm⁻¹. This work provides a robust and eco-friendly approach to the fabrication of high-performance and stable perovskite thick films for advanced radiation detectors.