Severe impedance mismatch problems due to the various environmental issues in downhole can greatly degrade the reliability of the radio-frequency identification (RFID) control system. In this brief, a new adaptive RFID impedance-matching system for oil well applications is presented, in which the measured real-time phase difference is employed to track the impedance mismatch information, while a variable capacitor array combined with a frequency sweep is used to rematch the mismatched impedance. Experiments have been performed and the results show that the impedance can be rematched perfectly when affected by from the various environmental influences.