In order to promote the construction of green mines and address core technical challenges in safe mining during the transition from the caving method to the open stope subsequent backfill method, it is imperative to scientifically determine the safety thickness of the isolation layer to ensure mining safety. Based on the fractured arch theory and the small deformation thin plate theory in elastic mechanics, this study calculates theoretical safety thicknesses of 12m and 14m through comprehensive analysis of orebody physical-mechanical parameters and stope structural parameters. Considering necessary redundancy, a comprehensive safety isolation layer thickness of 15m has been determined and continuously validated over six years of production practice. Research demonstrates that the dual verification method integrating these two theoretical frameworks can effectively determine the isolation layer safety thickness, ensuring roof stability in mined-out areas while maintaining safe and stable implementation of subsequent backfill processes for a 5 million tons/year mining operation. This technical approach establishes a replicable methodological framework for similar mines transitioning from caving methods to subsequent backfill methods.