In order to improve the reliability and quick response of relay protecti on device, the theory and requirement of the low-voltage relay protection device was analyzed. According to the features of relay protection device measurement and protection, project that was about low-voltage relay protection device for the features of measurement and protection based on C8051F347 was put forward. This design included the software and hardware of the electric energy measurement. It improved the performance of the display and input and designed communication and protection circuits and then to experiment the project. The result showed that the device had the characteristics with simple construction, high reliable action, high sampling accuracy, easy operation, rich interface and so on. It was concluded that the device can complete all function of relay protection in the complex situation.