Abstract: The flame-retardant polypropylene (PP) composites doped with a small amount of colemanite (CB) or zinc borate (ZB) were prepared with the melt blending extrusion method. The influence of CB and ZB on the combustion properties, thermal stability, mechanical properties and crystallization of flame-retardant PP composites filled with melamine-coated ammonium polyphosphate (APP-102) and melamine cyanurate (MCA) were studied. The results showed that a small amount of CB or ZB (2.0%) can effectively improve flame retardancy of PP composites, the LOI value were improved from 25.7% to 27.6% and 27.7% respectively, the UL-94 rating was increased from V-2 to V-0, and the peak heat release (pHRR) and total heat release (HRR) were reduced slightly. CB and ZB could effectively improve the thermal stability of flame-retardant PP composites, and boron could promote the formation of a continuous and compact char layer on the surface. The addition of a small amount of CB or ZB did not deteriorate the mechanical properties of the flame-retardant PP composites, and the tensile modulus and flexural modulus were slightly increased. At the same time, the crystallization rate and crystallinity of flame-retardant PP composites with a small amount of CB or ZB were improved. Therefore, CB was similar to ZB and could be used in synergistic flame retardation of PP.