Promising phosphorus-based flame retardants reduce thermal hazards while increasing non-thermal hazards. This work utilizes the oxidative defects of black phosphorus, which can synchronously reduce and stabilize copper(I) molybdate while introducing surface modification with sodium alginate in an environmentally friendly ball milling process to enhance dispersibility. First, the T5% and Tmax of PC/BP@CM@SA (2 wt%) increase by 9.5 °C and 8 °C, respectively, under nitrogen. Second, UL-94 V0 was achieved, and the peak heat release rate, total heat release, and total smoke release decreased by 44.0 %, 22.1 %, and 17.0 %, respectively. Compared with that of the neat sample, the height of the char layer doubled, while its graphitization degree decreased from 2.38 to 2.14. In short, this is largely attributable to the catalytic action and physical barrier effect of BP@CM@SA. This study provides a way to balance the reduction in thermal hazards and increase in non-thermal hazards of phosphorus-based flame retardants such as black phosphorus.