The simulation and design of X-band down-converter Field Effect Transistor(FET) mixer is presented. The basic principle of FET drain mixer is analyzed by the harmonic balance method and the transform matrix method. How to choose the appropriate FET transistor is based on the requirement for design. The circuit is designed，simulated and optimized with Advanced Design System(ADS) software. With Radio Frequency(RF) of 12.3 GHz-13.2 GHz and Intermediate Frequency(IF) of 1.6 GHz-2.5 GHz，the conversion loss is less than 5 dB.