An embodiment of the invention provides a digital subscriber line access system with a chair back structure, comprising two layers, wherein one lay is fixed with a subscriber board, and the other lay is fixed with a service treatment single board, which are connected by a back board, and the layer fixed with the service treatment single board is actualized by a small-sized line card comprising atleast one spare slot. By inserting a main control unit or extender cards with kinds of different functions in the spare slot in the system, the extensibility of the digital subscriber line access system with a chair back structure can be enhanced, wherein the extensibility comprises the aspects of service interface, service protocol processing function and fixed and mobile interinfiltrate treating function.