Two kinds of ammonium polyphosphate (APP) including untreated APP and surface-modified APP treated by a silane coupling agent were microencapsulated by MF resin. Then PP/MCAPP flame-retardant material was prepared. The results indicated that the coating of MF microencapsulated surface-modified APP was more uniform and complete, and its thermal stability was higher than that of MF-microencapsulated untreated APP. Furthermore, the LOI value of PP/MCAPP (MF-microencapsulated surface-modified APP) composites was 30 and it could pass the UL-94 V-0 test; moreover, the water resistance of the composites was better than that of PP/MCAPP (MF-microencapsulated untreated APP).