Two kinds of flame-retardant polymers prepared from the phosphorylation of poly(vinyl acetate) were employed to reduce the flammability of polypropylene (PP). The two flame-retardant polymers had very different effects because of the differences in their chemical structure. Effects on flame retardancy, thermal properties, mechanical properties, and phase morphology were investigated. Under a certain molecular design, the flame-retardant polymers were able to offer good flame retardancy, good mechanical properties, and good PP matrix compatibility.