Abstract: Active Structural Acoustic Control(ASAC) is an efficient method in acoustic radiation control of coupling enclosure. In the past research of ASAC, the concept of "Acoustic Radiation Mode(ARM) of coupling enclosure" was brought forward, which was a set of basic functions of structural mode amplitude. However, there was an incompatibility with the ARM definition in free space radiation case which was a set of basic functions of normal velocity or pressure on the vibrating surface. Also, there was severe inconvenience for application as structural modes were required while accurate and useful structural modes were difficult to be extracted in practice. To overcome these problems, by analogy to ARM theory of free space, the acoustic potential energy was expressed in quadratic form of normal velocity on coupling surface and ARM of coupling enclosure was redefined. Furthermore, theoretic deduction showed that ARM of coupling enclosure could be replaced simply by corresponding acoustic mode projection of enclosure when the coupling surface was discretized into equal size elements. Therefore, the ARM theory of coupling enclosure which was consistent with that of free space and convenient for application was formed. Finally, numerical calculation was performed and the results proved that the presented theory was very efficient in ARM calculation of coupling enclosure and ASAC.