Abstract: The paper deduces the formulas for designing and realizing fourth harmonic mixer and presents the mixer formula through combine the theory of anti diodes and the theory of composing differential phase signals. Based on the formula, the scheme of realizing Ka band high IF forth harmonic mixer is given and then the circuitsimulation is performed and the mixer circuit is worked out. The results of simulation and test show that the formulas are correct and the scheme is practicable.