In this paper,mathematical model and its uncertainties of a typical hydraulic steering system is analyzed.The control problem of the system can be come down to a combined H 2/H ∞ output feedback control problem.By transferring the dynamic output feedback problem into a static feedback control problem of an auxiliary system,the problem can be solved by linear matrix inequalities (LMI).Simulation results reveals that the H 2/H ∞ controller based on LMI is better than PID and H 2 or H ∞ controller