The nitrogen, phosphorus and zinc compounds in the water were adsorbed by the waste residue (WR) from the magnesium-based wet flue gas desulfurization process. Then, the zinc ammonium phosphate (ZAP) was synthesized in situ, and the modified WR (MWR) was obtained. MWR were characterized by XRD, FT-IR, SEM and TG. The results certify the formation of WR/nano-ZAP composites with the average grain diameter of ZAP about 8.16nm. The polypropylene (PP)/MWR flame retarding materials were tested by oxygen index (OI) and universal mechanical tester to study the fire resistance and mechanical properties of PP/MWR. The limit oxygen index (LOI) of PP/MWR (25 phr) is about 27.1%, and the flame resistance of PP/MWR reaches UL-94 V-0 level. The tensile strength of PP/MWR is about 6.95% higher than that of PP after 20 phr MWR added. MWR shows a good application prospect in the fields of waste water treatment and flame retarding materials.