A Schottky diode model for the mixer’s design is presented in this paper. According to the physical structure of the Schottky diode, a novel 3D electromagnetic (EM) model was built and analyzed by finite element method. A wide-band equivalent circuit up to 110 GHz was proposed taking the parasitic effects distributed in the diode into account. Based on the proposed equivalent circuit, a Ka-band balanced mixer was designed and optimized. The measured conversion loss is 7.5 dB to 10 dB from 32 GHz to 37 GHz, which agreed well with the simulated one.