The linear matrix inequality(LMI) robust control method is widely used in the design of helicopter flight control system. Current methods only consider the model uncertainty caused by gust and noise etc. The perturbation of state matrix due to flight state variation is neglected. In the paper, an improved LMI method considering both external disturbances and state matrix perturbation is established. The flight control law for attitude command attitude hold (ACAH) of Z-11 helicopter using the developed method is implemented. Finally, the simulation is carried out under the Matlab/Simulink environment. The results show that the control law designed by using the developed method has good performance and the improved robustness.