The analysis and optimizing design for three-port device drain mixer in Ka-band were given. The nonlinear equivalent circuit and its parameters were simulated and obtained from the small signal S parameter and the DC measurements of three-port device. And then, the drain mixer circuit was designed with optimum conversion gain of the drain mixer which was derived from the harmonic balance method and the conversion matrices method. The test result shows that the mixer has conversion gain 4dB at RF 27.4GHz and LO 33.4GHz/10dBm.