Decoupling control can substantially simplify the design of multivariable control systems. However, decouplers are usually sensitive to modeling errors, especially for the systems with relative gains bring far away from unity. Hence, decoupling control is limited in practical application. In the present study robust control of a decoupling system with external loops is developed. Design of the control system consists of two steps. The first step is to design the decoupler and the controller based on the nominal plant. The second step is to design the filters in the external loops by using a robust stability criterion. The robust control is implemented without changing the original design of the decoupler and the controller. The control system with external loops for decoupling control significantly improves the control performance. The proposed robust control enhances the applicability of decoupling control.