An airtight cosmetic powder box includes an outer box having an upper lid and an outer box body. The outer box body includes a receiving space receiving an inner box unit. The inner box unit includes an upper cover and an inner box body. The inner box body includes an inner receiving space. A seal ring is received in an annular groove in a top face of the inner box body. The upper cover is pivotable relative to the inner box body between an open position and a closed position. The upper cover includes an annular rib extending from a bottom face of the upper cover. The annular rib is received in the annular groove of the inner box body. The bottom face of the upper cover and an inner peripheral face of the annular rib sealingly contact the seal ring, providing an airtight cosmetic powder box.