This paper approaches the lithostratigraphy and the division of oil- and gas-bearing layer series about Middle-Upper Triassic Yanchang Formation in Ordos Basin.The aim is to supply theory ground and easy working method for developing Chang21 oil member accurately and high effectively.Chang21 oil-bearing layer member can be divided in detail and finely defined into three compound oil-and gas-bearing horizons and some single oil layers further.The development practice has proved that the scientific division and accurate collation of Chang21 oil layer member is the most basic and the most key study when the authors develop the oilfield in this area.