In order to solve the problem that schemes of preventing over-grade trip in power grid of coal mine can not satisfy the requirement of real-time, this paper proposes a scheme of preventing over-grade trip based on switched Ethernet. This paper analyzes the working principle of the protection system, makes a design of hardware for the intelligent interlock device, and finally conducts a real-time simulation of the preventing over-grade trip system. The results show that the scheme can meet the requirements of speed and selectivity of instantaneous protection to prevent the over-grade trip occurs.