A single-layer plastic composite board (20; 30; 40) is disclosed. The single-layer plastic composite board is a two-piece single-layer plastic composite board comprising a single-layer plastic sheet (11; 21) and a stiffener (22; 32), wherein the single-layer plastic sheet (11; 21) has a main body portion (111; 211) and a peripheral portion located around the main body portion; the peripheral portion bends towards the back of the main body portion in a direction perpendicular to the main body portion to form a bent portion (112; 212); and the stiffener (22; 32) is engaged with and fixed to the bent portion (112; 212) of the single-layer plastic sheet (11; 21). The single-layer plastic composite board has advantages such as a simple structure, small thickness, light weight, high strength, low cost and ease of manufacturing, and can be widely used as a cabinet board, a box board, a table board, a drawer board, a seat board and a backrest of a chair, etc.