This paper presents an early warning and preventive control system for transient stability. This system has been used in several provincial power grids. Firstly， the framework and the main modules of the proposed system are introduced. Secondly， the key technologies of the system are also presented. An algorithm is implemented to calculate total transfer capacity（TTC） under four different power increasing strategies. The preventive control algorithm， which is based on transient energy function sensitivity， is also realized. Thirdly， some engineering-oriented methods are introduced to solve practical problems， which emerge in real application. There are several examples: a node-breaker model is implied to adapt to the changing operation mode， a scheme is presented to avoid misjudge of stability in the case of island， generation areas and load areas can be automatically divided with new method.