Abstract: Polypropylene （PP） is flammable, has poor charring ability, produces molten droplets during the combustion process, and releases a large amount of heat and toxic fumes, thus causing fires. In addition, single hydroxide flame retardants have low flame retardant efficiency and limited smoke suppression capability. In order to address the above problems, dolomite is used as the raw material to prepare compound hydroxide flame retardant （MCH） by calcined heat preservation-steam hydration process, and MMCH is obtained by surface modification with stearic acid. The flame retardant properties and mechanism of action in PP matrix are systematically investigated by comparing with single magnesium hydroxide （MH）. The PP-based composites are prepared by stirring and blending, melt extrusion and injection molding, and their related properties are tested. The results show that: with the increase of flame retardant content, the hardness and modulus of PP/MCH composites increase more obviously, the introduction of MMCH improves the impact properties and elongation at break of the composites, and at the same time, PP/MCH composites have more microporous structure and higher moisture absorption rate. The equal proportion of different composites is added to the same proportion of MMCH, which results in a higher moisture absorption rate and an increase in the proportion of different composites. The thermal stability of PP/MCH composites is significantly higher than that of PP/MH composites when different flame retardants are added in equal proportions, and the highest carbon residue is maintained at the same time, and the carbon residue of PP-45MCH is as high as 34.03%. In the horizontal burning test, the burning rate of PP-45MH reaches 20.66 mm/min, and the burning rate of PP-45MCH reaches 20.00 mm/min. The pHRR of PP/MH composites was lower, but the smoke suppression effect was obviously inferior to that of PP/MCH and PP/MMCH composites.