A kind of modal expansion forms based on acoustic waveguides modal is presented to investigate the vibro-acoustic coupling performance of the sandwich structures and its weak coupling forms is also developed. The boundary of the gap included two forms of pressure released boundary and rigid boundary. In comparison with structural modal expansion method and acoustic modal of the cavity, the influence of the orders of acoustic modal and acoustic waveguide on the calculation accuracy of the sound transmission loss is analyzed and the vibro-acoustic coupling performance and sound pressure distribution of gap were studied. It is shown that the needed number of the acoustic waveguide modal and acoustic modal of the cavity are determined by the highest frequency of interest; the bigger the highest frequency is, the more the number of the acoustic waveguide modal and acoustic modal of the cavity need. The comprehensive effect of structural vibration modal and acoustic modal act on the sandwich structures and affect its vibro-acoustic coupling responses. The ‘beam-air-beam” resonance, standing-wave resonance and structural resonance reduced the sound insulation performance of the structure. The boundary of the gap affects the vibro-acoustic of the structure obviously. The sound pressure distribution is different for the same physical phenomenon with three different methods and reveals the mechanism of the physical phenomena.