In this paper, we obtain a general equation for determining the resonant frequencies of all the modes in an open resonator in the form of waveguide with slow-varied cross section, which has any shape of longitudinal profile. The method to solve the field longitudinal distribution function in the resonators is investigated. As examples, we derived formulas for determining the resonant frequencies of all the modes for two kinds of resonators that have drum shape and dual-cone shape intersurfaces.A good agreement between the theoretical analysis and the experimental results has been demonstrated.