Abstract: Model reduction and active control of a flexible plate with dense frequencies are investigated. Firstly, dynamic model of the plate is established using the finite element method; then the balanced reduction method is applied to get a lower-order model; finally an active controller is designed using the optimal control method. The balanced reduction method is numerically compared with the modal cost analysis (MCA) method. Simulation results indicate that, the balanced reduction method is effective in reducing the degree of freedom of the plate, and is applicable for dense-frequency case.