The design principle for a S band low noise amplifier are presented. The key factors which affect the stability, noise figures, transmitted power of low noise amplifier(LNA) are analyzed. The EDA(Electronic Design Automation) software of advanced design system(ADS) is applied to design and simulate a 2 stage amplifer.The simulation results show that the LNA achieves a power gain of (26.2±0.5)dB as well as a maximum noise figure of less than 0.5 dB,input VSWR better than 1.4,output VSWR better than 1.14,being unconditionally stable at the frequency range from 2 150 MHz to 2 400 MHz.