Flexible ring net barriers have become increasingly popular in practical mitigation of debris flow worldwide. Systematic assessments of their response and performance subjected to the impact of realistic debris flows remain challenging. This study presents a novel computational approach based on coupled CFD-DEM to model the impact of debris flow on a flexible ring net barrier in a unified framework. The debris flow is treated as a solid-fluid mixture, where the solid phase and the fluid phase are modeled by DEM and CFD, respectively. The barrier is simulated as a system of different deformable components, including rings, cables and energy dissipators, and is modeled with DEM. The proposed method expedites a convenient, unified consideration of multi-way interactions among the debris solid, the debris fluid, and the barrier. The simulation of a flexible barrier is calibrated against existing experimental data and past numerical results, by examining the quasi-static responses of different barrier components and the dynamic reactions of the entire barrier. The barrier system is further subjected to the impact of debris flows with different Froude numbers to examine its performance, in terms of its retaining capacity of debris mass and peak sustained forces in the barrier. Two energy-related indices, energy dissipation ratio and energy absorption ratio, are estimated for design reference. The study provides a novel, physically based predictive computational tool for future design and analysis of flexible ring net barriers in debris flow mitigation.