The study of the characteristics of noise sources in cylindrical shells is the foundation of sound field prediction. Although noise sources are usually regarded as point sources to simplify the calculation model in noise source localization and waveguide sound propagation, the approximation is limited to far-field problems. For the near-field acoustics problems in engine room and ship cabin, the radiated noise possesses the spatial directivity because of the complex vibration distribution of the noise source surface. Moreover, the sound scattering on the surface of finite-size sources makes the noise source itself act not only as the energy input of sound field, but also as the scatterer to change the structure of sound field in the environment. These factors lead to large errors when the finite-size source is simplified into a point source. In order to explore the influence of finite-size source on the acoustic field inside and outside the underwater vehicle structure, the shell coupled equation is constructed by combining thin shell theory, equivalent source and Green function. The effects of source surface scattering and directivity on the acoustic field inside and outside the cylindrical shell are studied. The results show that the accuracy of finite-size source construction is related to the equivalent source location. It proves that equivalent source allocation should be arranged in the middle of the geometric center of sources and its structural surface. Sound scattering from the surface of the finite-size source will change the sound field inside the shell, and then the resonant peaks of the cavity are shifted to the high frequencies as the source volume increases, which causes a strong sound transmission phenomenon in some frequency bands. In addition, the directivity of the finite-size source has little effect on the intensity of the sound field inside and outside the shell, which is evident in changing the far-field directivity of the radiated sound field. The research results are valuable for noise prediction and noise control.