The invention discloses a low-cost wood-plastic composite material and a preparation method thereof. The low-cost wood-plastic composite material comprises, by weight, 40-60 parts of corn stalks, 25-40 parts of sisal dregs, 35-50 parts of abandoned plastics, 15-20 parts of polyvinyl chloride, 3-5 parts of aramid fibers, 5-7 parts of cotton fibers, 6-9 parts of calcium stearate, 2-3 parts of antioxidant, 2-4 parts of light stabilizer and 3-5 parts of magnesium hydroxide fire retardant. The preparation method includes the processing steps of modified pretreatment, high-speed material mixing, hot press molding, cooling and shaping. According to the low-cost wood-plastic composite material and the preparation method thereof, the corn stalks, the sisal dregs and the abandoned plastics are comprehensively used as main raw materials, so that cost is lowered, and the environmental pollution problem of waste is further solved. The obtained wood-plastic composite material has the advantages of being high in compressive strength, high in abrasion resistance and the like. The wood-plastic composite material further has the advantages of being simple in processing technology, short in processing time, low in cost and the like, thereby being suitable for industrial production.