This paper put forward a control system of static synchronous compensator (STATCOM) based on the EtherCAT bus technology, which had the characteristics of flexible expandability, Flexible information allocation function, high transfer rate, very short data exchange time, and accurate synchronized clock. The control system used two EtherCAT bus nets: main control net and module net. The main control net adopted series communication mode, and had data sample slave, data input and data output slave, regulation and protection slave, and transient fault record slave in the main control net. The module net adopts stelliform communication mode, and all slaves are derived from the couples. There are regulation protection slave, transient fault record slave, and all module slaves in the module net. The regulation protection slave and the transient fault record slave belonged to two EtherCAT bus nets. Digital signals processor (DSP) was used in the regulation protection slave, the transient fault record slave and the module slaves, which could improved signal transmission and control speed, and meet the requirements of real-time control. The experiment results show that the control system can significantly increase the device expandability, allocation function and transfer rate, achieve precise synchronization. The control system can be applied in the chain STATCOM and other flexible AC transmission systems (FACTS) used chain converter, which has the characteristics of clear control purposes, easy to implement and stable control effect.