Abstract: A PIN diode sub-circuit model is used to analyze I region conductivity modulation mechanism excited by microwave power. The time-frequency response of a PIN diode effected by I layer thickness and minority carrier lifetime is studied with ADS transient, harmonic simulation. The results show that: under a identical excitation, the thicker I region is, the more the spike leakage power is, the little the isolation is, and the longer minority carrier lifetime is, the shorter the spike leakage pulse width is, the more the isolation is.