Detailed airgap permeance information is of great importance for analyzing the operation principle and optimizing the electromagnetic field distribution of an electric machine with unconventional topology. So far, analytical approaches upon calculating airgap permeance have been adopted in most existing research work, while corresponding simulation-based tools have rarely been reported. In this paper, a new concept of airgap permeance calculation developed from numerical finite element tools is presented. Based on similarity of magnetic field and electric field, the airgap permeance distribution of electrical machines with various topologies can be predicted rapidly through finite element analysis (FEA) in electrostatic field. The FEA-based airgap permeance distribution can be regarded as an effective approach to validate corresponding results calculated from complex mathematical derivation. Furthermore, the characteristics of working airgap flux density harmonics introduced by magnetomotive force (MMF) of stator or rotor could be more intuitively investigated. Based on case studies of several typical machine topologies, the proposed FEA-based method will be proved of featuring wide feasibility, i.e., capable of covering stator slotting effect, rotor saliency as well as dual side saliency effect. Finally, the reasonability of the permeance calculation method is verified based on a further proposed homopolar topology within electromagnetic FEA.