Disclosed is a security monitoring method determining whether a trespasser is detected and a position of the trespasser in a set security space and monitoring sound generated at the position of the trespasser by using an acoustic image generated from acoustic signals generated by an acoustic generating device and an acoustic measuring device in an array type. An exemplary embodiment of the present disclosure provides a security monitoring system including: an acoustic generating device that generates acoustic signals; a plurality of acoustic measuring devices that receive the acoustic signals; and an acoustic image processing device that generates an acoustic image using a beamforming algorithm from the acoustic signals received in the plurality of acoustic measuring devices and determines a position of a trespasser by comparing the acoustic image after the trespasser is detected with the acoustic image before the trespasser is detected.