Abstract: Modern aircraft to achieve the maximization of the RF stealth performance in the future requiring shorter airborne radar boot time, which makes it possible of a low RCS stealth design for phased array antennas during airborne radar turn off. PIN diode is presented to reduce radar cross section (RCS) of microstrip antenna when it doesn't work. PIN diode is equivalent to resistance and capacitance when diode is forward-biased and reverse-biased, respectively. For the plane wave with different incident states, the bias state of PIN diode is determined according to the induced electric field distribution. And equivalent resistance value of PIN diode in the forward-biased state is optimized. As a result antenna RCS is significantly reduced. Simulated results show that optimization of PIN diode state on the basis of electric field distribution of current incident wave can dramatically reduce antenna RCS, and the maximum RCS reduction is more than 25 dBsm when antenna doesn't work. While the radiation performance of the antenna is not affected when it works.