Nowadays, various servo controllers on the market are increasingly pursuing higher power density, which means that the hardware design of each module in the system should be small enough. The application of EtherCAT communication technology has become the mainstream of the development of servo system interface. Therefore, this paper designs a small-size EtherCAT slave controller applied to the servo system. By using ARM to simulate the memory function of EEPROM, sharing the clock source between the slave communication element and the servo control element, and integrating the slave communication module with the servo control module, the volume of EtherCAT slave hardware platform is reduced effectively, which provides favorable conditions for improving the power density of the servo system. After experimental verification, the final design of the small-size EtherCAT slave controller can perform standard, correct and reliable EtherCAT communication work.