A SINR-based intrusion detection system comprises a first antenna, a second antenna, and an arithmetic unit connected to the second antenna. The first antenna simultaneously transmits one or plural beams having mutually different frequencies in an enclosed space so as to cover the enclosed space. The second antenna receives signals of the one or plural beams so as to capture signal parameters of the beam. The arithmetic unit is connected to the second antenna and detects the signal parameters in an always-on manner so as to obtain a SINR value from the signal parameters, comparing the SINR value to a predetermined threshold range, and determining that there is object intrusion when the SINR value falls within the predetermined threshold range.