This invention discloses a method for smooth capacity expansion of data communication product, and a smooth capacity expandable system of data communication. Architecture of the system at least comprises circuit card and switched network card; and further comprises interface card of switched network, interface transfer card and connection optical fiber. Based on these, it provides one kind of multiple frameworks structure with interconnection between frameworks. When the capacity is expanded, the original circuit cards and switched network cards keep unchanged, only smoothly increases the switched network cards and numbers of circuit card frameworks. Therefore, it implements smooth capacity expansion and client investment protection at the same time. With adding backup cards, it also implements capacity expansion without interrupting the service.