In order to solve the deficiency that traditional user-defined indicator does not consider the internal structure of multivariable decentralized controller, we propose the assessment method that based on improved user-specified benchmark. In the method, the open-loop delay and interference of traditional user-specified benchmark replaced by the effective partially closed-loop delay and interference correspondingly. New right diagonal interactor matrix replaces the unit interactor matrix, which greatly simplifies the complexity of solving performance indicators. The experimental results prove that the method can assess decentralized control system with internal interference and is effective for performance assessment