Abstract: Annulus cathodes with structural field enhancement which can improve the electron emission performance effectively have wide application in load of pulsed hard X-ray, preparation of strong electron beam and high-power microwave system. It’s of great significance to make investigation on structural field enhancement of annulus cathode. The charge equivalent method and mirror image method were applied to build the model for calculating the electric field distribution between cathode and anode. The electric field distribution of cathode surface and the macroscopic field enhancement factor were calculated with the theoretical model. The practical estimation formula of the macroscopic field enhancement factor was obtained. The calculated results show that when d/w (the ratio of distance between cathode and anode to the width of cathode ring) is less than five there is a good linear positive correlation between the macroscopic field enhancement factor and d/w, and the theoretical result is consistent with simulation result of Ansoft.