Abstract: In the article, we proposed a underground short circuit protection system based on Ether CAT substation, which mainly composed of Ether CAT substation and integrated protection device with Ether CAT communication interface and interlock interface. The Ether CAT substation supports Ether CAT and fiber interlock communication mode. Ethernet Ether CAT and optical fiber interlock network can cooperate with each other, when one is interrupted by the failure, the other network can complete the work of system independently. The system can achieve short circuit protection selectivity, determine the position of fault quickly, shorten troubleshooting time and improve safety and reliability of power grid system.