Abstract: Vibration prediction for stern cabins is an important part of research into the global vibration of ships. To address the need to improve precision and efficiency, a study is carried out on the proportion of a mixed model and the length of a 3D stern cabin model to investigate the natural frequency of a ship. Using the FE method, different types of model are established for a research vessel, including six mixed models that are used for the basis of a detailed discussion on the different proportions of mixed models, and five stern cabin models that are used to analyze the impact of the modeling range on the natural frequency. Moreover, the Lewis method and virtual mass method are both used to consider the impact of outside water in the analysis of the wet mode. It is observed that the results of the calculated natural frequency of the mixed models are approximately the same when the proportion of the mixed models is over 1/5 the length of the ship, and in good agreement with the results of the calculations when the modeling range of the stern cabin models is exactly in the vicinity of the vibration node of the ship. For the local vibration calculation of ships using the Lewis method and virtual mass method, the results of various schemes differ greatly when applied to the stern cabin models. The results suggest that the proportion of local cabin models should be 1/4 the length of the ship in the natural frequency calculation, and the virtual mass method is recommended for considering the impact of local additional water mass.