Model reduction and active control of a flexible plate were investigated and experimented to verify the theoretical result. Firstly, the dynamic equation of the plate was presented using the assumed mode method. Then model reduction was studied using the modal cost analysis (MCA) method. Considering the system with small damping, a simplified approach for the MCA method was presented.An active controller was designed using the classical optimal control method. Simulation and experimental results indicate that the MCA method is able to demonstrate the relative importance of all the modes of the plate, so it can be used for model reduction of the plate.