It introduces a simple and robust polarization-insensitive phase modulation method that is suitable for a plug and play quantum key distribution system, with a Faraday mirror and polarization sensitive phase modulator. The pulse is modulated twice acorrding to its polarization before and after it is reflected by a Faraday mirror. Preliminary experiments at 1550 nm wavelength demonstrate an interference visibility as high as 96%, regardless of the polarization of the signal.