Disclosed are a passive RFID (Radio Frequency Identification) system and method capable of recognizing a great deal of tags at a high speed. A reader of a passive RFID system, including: a processor dividing slots and subchannels to configure a round, recognizing subchannels that are not involved in a collision and interference by the slots, and changing the subchannels that are not involved in a collision and interference, into a sleep state; a transmission unit encoding a command generated through the processor, performing an amplitude jitter modulation on the encoded command, and transmitting the modulated command; and a reception unit acquiring and demodulating tag signals received through a plurality of subchannels by the slots, and providing the demodulated tag signals to the processor.