Wood-plastic composite (WPC) is a kind of renewable building material, but its poor flame retardancy limits its safe use. In order to improve the flame retardancy of WPC composites, the compound flame retardant composed of modified ammonium polyphosphate (M-APP) and expanded graphite (EG) was added into the WPC matrix to prepare WPC composites. The mechanical properties, flame retardancy and thermal stability of WPC composites were tested. Compared with unmodified WPC, when the ratio of M-APP to EG is 1:1, the LOI value of WPC/A₁/E₁ increases by 44.4%, the peak heat release rate decreases by 80.1%, and the total smoke production decreases by 91.6%. The tensile strength of WPC/A₁/E₁ is 25.1 MPa, which is 3.7% higher than that of unmodified WPC. The flexural strength of WPC/A₁/E₁ is 45.1 MPa, which is 1.1% higher than that of unmodified WPC. The carbon residue of WPC/A₁/E₁ is 27.5%, which increases by 76.3% compared with unmodified WPC. When the ratio of M-APP to EG is 1:1, WPC/A₁/E₁ has better comprehensive properties.