Abstract: It is of great significance to study the acoustic scattering characteristics of complex underwater target with cavity structural anechoic coating layer. Firstly, the complex underwater target model is established by modeling software. Then, the cavity with variable cross sections is divided into several thin segments that can be regarded as the cylindrical cavity structure. The acoustic reflection coefficient is calculated by using the segment approximation method and the transfer matrix method of stratified medium. Finally, the target strength of the model is calculated according to the planar element method based on the Gordon integral. In addition, a ray-casting-based visible facet discrimination method is proposed for complex targets. Taking the BeTSSi Ⅱ model as an example, the influence of the incident azimuth and frequency of the acoustic wave on the acoustic scattering characteristics of the model is studied. The results show that the ray-casting-based visible facet discrimination method proposed in this paper can quickly and effectively provides the visible facets required by the planar element method; the calculation results of the BeTSSi Ⅱ model by the planar element method can well reflect the characteristics and regularities of its target strength; laying a cavity structural anechoic coating layer on the model can effectively reduce the target strength. Furthermore, compared with laying a uniform anechoic coating layer of equal thickness, the frequency of the first valley of the acoustic reflection coefficient curve of the composite structure decreases, which results in better sound absorption at low frequencies and can further reduce the target strength. The research in this paper is helpful for the realization of rapid prediction of the scattering characteristics of underwater platform in practical engineering.