Technical fields of building external wall decoration and material manufacturing, providing a mildewproof and antirot high-strength cement particle board and a preparation method thereof. The preparation method includes: (1) sequentially carbonizing and water-washing a shaving, and mixing the obtained carbonized shaving with a cement gelling agent, a curing agent aqueous solution and water to obtain a mixture; (2) molding the mixture to obtain a pre-molded material; and (3) sequentially curing and drying the pre-molded material to obtain the mildewproof and antirot high-strength cement particle board. Compared to ordinary cement particle board, which is not subjected to carbonization treatment and water-washing, the cement particle board of the present invention can effectively avoid mildew and rot, and can significantly improve the mechanical strength and durability thereof, helping to extend the service life of the cement particle board.