Abstract: In the preliminary stage of the multi-cabin spacecraft development, it is often a problem how to assign the requirement of the vibrational fundamental frequency for each cabin and confirm the satisfaction of the global requirement advanced by launch vehicle designers. However, as the detailed design parameters of each cabin are not yet available in this stage, there is no necessary input to build a three-dimensional finite elements model of spacecraft and this will also be time-consuming, to meet the requirement in the design iteration and optimization. For solving the problem, a rapid calculation method of the vibrational fundamental frequency assignment for the multi-cabin spacecraft is proposed. A multi-section rod model is used to simulate the longitudinal vibration, and a multi-section beam model is used to simulate the transverse vibration. The mathematical relationships between the design parameters of the spacecraft cabins and the beam-rod model parameters are established based on the equivalence principle of the fundamental frequency. The vibration frequency equations of the multi-section rod model and the multi-section beam model are derived, respectively, to achieve a rapid calculation method of the vibrational fundamental frequency assignment for the multi-cabin spacecraft. The method is verified by comparing its results with the modal analysis results obtained by using the three-dimensional finite elements model and the mechanical test results in the sub-sequent stage of the spacecraft development. The proposed method confirms the validity of the fundamental frequency assignment, and makes it feasible for the design optimization.