The input-output energy decoupling is put forth for the complex electromechanical tension system problem including strong coupling, multi-variance, uncertainties, etc. The aim is that an energy of any input controls mainly the energy of a corresponding output and influences the energy of the other as weakly as possible. After the looper height and tension control system has been modeled, a H_∞ robust control algorithm based on input-output energy decoupling is proposed in terms of the solution of linear matrix inequalities (LMI). Finally, the input-output energy decoupling algorithm is applied to the tension system, and the simulation results show that the method has satisfactory decoupling performance. The validity of the designed controller is validated.