Abstract: The design of fixed-order H_∞ controller, under the structure of full information, is investigated in this paper. A fictitious input related to controller parameters is introduced to guarantee the problem of full information structure. Because the augmented system is a singular H_∞ control problem, we treat this problem via a constant dependent Riccati equation. Under the constraints of H_∞ norm, the fictitious input takes the form of feedback of disturbance and state, then we can obtain the existence condition of the controller parameters.