The controller design determines dynamic and static performance of the FACTS devices. With the FACTS devices widely used in the power system，the interaction effect of multiple controllers in one FACTS device should be considered in order to make the power system more stability. The interaction between the damping control loop and voltage control loop of one FACTS device in single machine infinite bus power system is analyzed. Based on the analysis model of the system, the interaction of the two controller in STATCOM and SVC existed or not is expounded. Finally, the results are verified by BPA software simulation.