the invention relates to a polarization - controlled encoding method , encoder and quantum key distribution system , which is characterized in that polarization maintaining light path or 90 degree rotation faraday mirror are used inside the encoder to keep the polarization of the output pulses same , and that in the quantum key distribution system employing the polarization - controlled encoder the pulse emitted from transmitter is unidirectional - transmitted to receiver and then quantum key distribution is implemented using interference in the pulses according to the quantum key distribution protocol . the quantum key distribution system using the polarization - controlled encoder of the invention has the ability of avoiding the wiretapping to transmitter , receiver and quantum channel . detection units each of which separates reversed photon from other photons are added at the out port of the transmitter and the in port of receiver , respectively , so that trojan horse is prevented from entering and photons with phase modulated information are prevented from leaving the safe area in receiver . unconditionally safe key distribution can be accomplished by using the quantum key distribution system of the invention .