Abstract: The intumescent flame retardant (IFR) composed of piperazine pyrophosphate (PAPP) and melamine cyanurate (MCA) can improve the fire safety of polypropylene (PP). However, the addition of up to 28wt% PAPP-MCA (mass ratio of 3∶1) greatly reduced the strength and toughness of PP (by 31% and 62%, respectively). This article uses a rare earth compound cerium phenylphosphonate (CeHPP) with a two-dimensional layered structure as a flame retardant synergist to enhance the fire safety of PP with relatively low PAPP-MCA content. The results showed that in the presence of 2wt% CeHPP, adding only 20wt% PAPP-MCA can achieve a 30% limiting oxygen index (LOI) of PP and pass the UL-94 V-0 grade. At the same time, the peak heat release (PHRR), total heat release (THR), and total smoke release (TSR) of PP composite materials are significantly reduced by 82%, 85%, and 52%, respectively. In addition, compared with PP composites with the addition of 28wt% IFR at the same V-0 level, the tensile strength and elongation at break of CeHPP-IFR synergistic flame retardant PP composites increased by 22% and 110%, respectively, demonstrating more ideal mechanical application potential.