This paper studies the active structural acoustic control (ASAC) in a rectangular enclosure by applying different control forces to the elastic plate. The acoustic cavity model of a rectangular enclosure consisting of two simply supported flexible plates is developed to analyze the structural-acoustic coupled system of the enclosure. The formula is derived to calculate the sound pressure in the enclosure when some point forces are applied to the flexible plate. The optimal control model of ASAC is established and analyzed. Then the simulation of the sound pressure level (SPL) in the local cavity sound field when some of the second forces with different parameters are applied to the flexible plate is analyzed. Then, a simulated model to analyze the control effect on the partial acoustic field is developed. The results indicate that the parameters of the second forces to be given are more important to the control effect; but it is not better with an infinitely increasing number of parameters. Meanwhile, the number of second control forces isn′t better.