Abstract: Ammonium polyphosphate (APP) was encapsulated with formaldehyde-melamine resin and epoxy resin in a monolayer or double-layer mode and then used for flame-retardant modification of polypropylene (PP). The structure and performance of the resulting APP and relevant flame-retardant PP were characterized by Fourier transform infrared spectroscopy, scanning electron microscopy, horizontal vertical burning experiment and limiting oxygen index measurement. The results indicated that the water solubility of the encapsulated APP decreased, and its expansion ratio and flame-retardant efficiency were improved. The double-layer-encapsulated APP has lower water solubility and better flame-retardant effect than the mono-layer-encapsulated one. Compared to pure APP, the encapsulated APP had less effect on the mechanical properties of PP and stronger interface bonding with PP matrix.