In order to meet the fire safety requirements of materials in the fields of cables, electronic appliances, etc., the research of flame retardant polypropylene composites has been widely concerned. Aiming at the flame retardant effect of aluminum diethylphosphinate (ADP) on polypropylene (PP), the article used a dry modification method to modify the surface of ADP with silane coupling agents A-151, KH-550, and KH-560, explores their influence on the mechanical and flame retardancy of PP. The results show that under the same amount of addition, the modification of A-151 with vinyl functional group can improve the compatibility of ADP and PP, and improve the mechanical properties and flame retardancy of the composites, the limited oxygen index(LOI) of PP/20ADP is only 20.0%, and that of PP/20VADP is 27.3%, with an increase of 36.5%. Among the three modified ADPs, the composite with KH-560 modified ADP has the lowest peak heat release rate (pHRR) and total heat release (THR) when the adding mass fraction of flame retardant is 20%, compared with PP/20ADP, the pHRR is increased by 2.2%, but the THR is decreased by 10.5%. The reason is that the epoxide functional group carried by KH-560 can play a synergistic flame retardancy with ADP, thus improving the flame retardancy. In this paper, the flame retardancy mechanism of silane-modified ADP on PP is revealed with high silane addition amount.