With the gradual maturity of digital substation technology， the function and combining mode of intelligent electronic devices（IEDs） may have plenty of room for changes. A schema for the realization of a system protecting device is proposed based on data sharing and unified data source in the digital substation. Also discussed are its hardware design， software functions realization，adaptability and reliability in application. As system protection is the organic integration of many decentralized monitoring and control function modules in the digital substation， the system data flow is optimized and simplified. Owing to its low cost， easy maintenance， high performance and high reliability， it can adapt to the future development of power system automation and market demand.