Ammonium polyphosphate (APP), which is an important component of intumescent flame retardant (IFR), has been modified with (3-aminopropyl) triethoxysilane (KH550) and silicon resin to enhance its hydrophobic properties. The structure and properties of the modified ammonium polyphosphate (MAPP) were well-characterized by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy (SEM), and water contact angle (CA). The water CA of MAPP was as high as 148 degrees, which demonstrated that MAPP possessed excellent hydrophobic properties. MAPP and APP were mixed with triazine, which is a char-foaming agent (CFA), and polypropylene (PP) to prepare PP/MAPP/CFA and PP/APP/CFA composites, respectively. The fire retardancy and thermal degradation behavior of PP/MAPP/CFA and PP/APP/CFA composites were investigated by limiting oxygen index (LOI), vertical burning test (UL-94), cone calorimetry, and thermogravimetric analysis. The surface and fracture morphologies were evaluated by SEM. The mechanical properties was analyzed by tensile, flexural, and izod impact tests. The results demonstrated that the MAPP imparted excellent flame retardancy of PP/MAPP/CFA systems, along with a higher LOI value, UL-94 rating, and lower heat release rate and total heat released values. The modification enhanced the interfacial adhesion, mechanical stability, and thermal stability of PP/MAPP/CFA composites. Moreover, PP/MAPP/CFA composites with the thickness of 1.6 mm can still pass the UL-94 V-0 rating after treatment with water for 168 h at 70 degrees C, meanwhile the weight loss rate was decreased from 2.31% for PP/APP/CFA composites to 0.85%, indicating excellent water resistance. This investigation shows a promising formulation for water-resistant intumescent flame-retardant PP composites with extraordinary properties.