Railway transportation hold a leading post in the long-distance passenger transport market. Passengers spend most of their time sitting, so seat affect passenger comfort most among interior facilities of coach. One of the major tasks of interior design is functionality and comfort of seat. Seat width, seat depth and seat height are basic functions factors of seat. The seat depth is major issue of functionality design of 25g car. The shape of cushion and backrest are main comfort factors of seat. The backrest angle is major issue of comfort design of 25g car now. The interior system design of railway passenger car can be developed if we stress and make rational use of these factors of seat design.