In this paper, a unified method is proposed for the first time to predict the free and forced vibration behavior of the combined conical–cylindrical shells coupled with annular plates in the steady-state thermal environment. The energy equations of each substructure are obtained based on the first-order shear deformation theory (FSDT), and the artificial spring technique is introduced to realize the coupling between each substructure, the circumferential closure of each substructure, and the arbitrary boundary conditions. The displacements of the midface in each sub-structure are expanded using the spectral-geometry method (SGM), and the energy expression is solved by using the Rayleigh–Ritz method to obtain the free and forced vibration behavior of the combined structure. The numerical examples derived from the calculations are compared with the results obtained by the finite element method (FEM) to verify the accuracy of the present method. The effects of boundary conditions, temperature and geometric parameters on the free and forced vibration behaviors of the combined structure are investigated. The method in this paper is a meshless method, which is faster and more efficient as it is less dimensional and less computationally expensive than the FEM. This paper provides a compelling reference for vibration control of the combined conical–cylindrical shells coupled with annular plates in practical applications.