The invention provides a composite particle board and a preparation method thereof. The preparation method comprises the steps of (1) preparation of a base material, wherein the base material comprises bamboo shavings and straw fragments, the thickness of the bamboo shavings is 0.3-1.2 mm, the length of the straw fragments is 0.5-5 cm, and the moisture content of the bamboo shavings and/or the straw fragments is 5-13%; (2) mixing of the base material, wherein the bamboo shavings and the straw fragments are mixed to obtain an upper surface layer mixed material, a lower surface layer mixed material and a sandwich layer mixed material, and the mass ratio of the bamboo shavings to the straw fragments in the upper surface layer mixed material and the lower surface layer mixed material is 1:9-4:6; (3) sizing, wherein the three mixed materials and adhesives are mixed to obtain a sized mixed material; and (4) the sized mixed material is subjected to laying, hot pressing and after-treatment to obtain the composite particle board. Through the method provided by the invention, existing bamboo materials and straw resources are fully utilized; and the prepared composite particle board is good in mechanical property and good in dimension and has the moistureproof properties.