Embodiments of the application provide a monitoring method, server and system and an image acquisition device capable of obtaining a first distance ratio of the horizontal distances from three WiFi detectors to a target person. The application is also capable of obtaining a second distance ratio of the horizontal distances from the three WiFi detectors to a WiFi terminal based on the signal strength of WiFi signals. The application determines the MAC address of the WiFi terminal of the target person by matching the second distance ratios of various WiFi terminals with the first distance ratio, and monitors the target person according to the MAC address. Thus, the application achieves continuous monitoring of the target person.