Barrier dams formed by the large-scale debris flow that blocks the river often are subjected to a high risk of break. The catastrophic flood arising from dam break has been confirmed as the most serious hazard-causing body of the debris flow dams. A novel method is proposed in this study to regulate the discharge of the debris flow dam break based on a flexible protective net (FPN). Next, 17 sets of flume experiments were performed to investigate the effects of different inflow discharges, dam grain size distribution, and downstream boundary locations of the FPN on discharge regulating effectiveness, such that the flow regulating mechanism of the FPN was revealed. As indicated by the results, (1) a significant traceability scarp was formed in the debris flow dam-break process. The whole dam-break process fell into three stages, i.e., traceability scarp formation, traceability scarp erosion, and traceability scarp decline. (2) When the downstream boundary of the FPN was located close to the downstream dam toe of the debris flow dam, regardless of the inflow discharge and the grain size distribution of the dam, the FPN laid on the surface of the debris flow dam took on critical effect in regulating the outburst discharge. (3) The inhibitory effect of the FPN on the particle initiation and the interception effect on particle transport, as well as the energy dissipation effect of the FPN and coarsening layer on outburst flood, fundamentally accounted for the good flow regulating effect of the above-described method. The method proposed in this study is capable of achieving an ideal outburst flood regulating effect by ensuring construction safety.