In this work, the effect of both the expandable graphite (EG) and ammonium polyphosphate modified with 3-(methylacryloxyl) propyltrimethoxy silane (M-APP) on the flame retardancy and mechanical properties of the wood-polypropylene composites (WPC) were studied. Cone calorimetry results indicated that both EG and M-APP could effectively improve the flame retardancy of WPC, while the retardancy of EG was better than that of M-APP. When the flame-retardant loading was 25 wt.%, the limiting oxygen index (LOI) values of M-APP-filled WPC (M-APP/WPC) and EG-filled WPC (EG/WPC) were 30.7% and 37.9%, respectively. According to the LOI test, the optimal ratio of M-APP to EG in WPC was 1:1 by weight, at which the LOI value of WPC was 39.3%. Thermogravimetric analysis (TG) results indicated that the addition of M-APP and EG to WPC could lead to an increase of char residue. Under the same conditions, the char residue of composite filled with the mixture of EG and M-APP (at a ratio of 1:1) was greater than that of composites filled individually at the same temperature. Both the tensile strength and flexural strength decreased at a certain extent due to incorporation of EG into WPC, but with the addition of the M-APP, the mechanical properties of these composite samples increased. At the ratio of 1:1 (M-APP to EG), the mechanical properties of the composite were not obviously decreased, and the flame retardancy was higher than the M-APP-filled WPC composites.