The impact of SSSC on the power flow is studied under dq reference frame firstly. Then the different functions of the d -axis and q -axis components of SSSC output voltage are analyzed. The power flow control characteristics of SSSC are studied and then the constant power flow control approach of SSSC is proposed. The SSSC output voltage is decoupled by using state feedback exact linearization approach. Then the power flow of transmission line and the DC voltage of voltage source converter can be controlled independently. Through the two PI controllers， the d -axis and q -axis components are used for controlling DC capacitor voltage and power flow in the transmission line respectively， which can make the DC capacitor voltage and power flow track the expected value. In addition， the PI control can also improve the leak robustness caused by state feedback exact linearization approach. Finally， digital simulation results show the effectiveness of the proposed control approach.