The invention discloses a low-cost bamboo-plastic foam composite door-window material and a preparation method thereof. The preparation method has the advantages that rice straw is soaked by using alkali liquor, and is treated by using hydrogen peroxide; on one hand, lignin in the rice straw can be melted away and the preparation is made of a later reaction, and on the other hand, the structure and the performance of a rice-straw fiber can be also ameliorated and the interface bonding between the rice-straw fiber and a composite material is improved; therefore, the hardness and the compression strength of the composite material are improved.