A vital-sign detecting system includes radio-frequency (RF) tags disposed on detected subjects respectively, one of the RF tags being turned on and generating an incident RF signal with a predetermined frequency, and the incident RF signal projecting on a corresponding detected subject to generate a corresponding reflected RF signal; and at least one radio-frequency identification (RFID) radar that turns on one of the RF tags, demodulates the reflected RF signal to obtain vital signal of the corresponding detected subject, and identifies the detected subject according to the turned-on RF tag.