The present application provides a monitoring method and device for a mobile target, a monitoring system and mobile robot. The present application through the technical solution that acquiring multiple-frame images captured by an image acquisition device under an moving state of a robot in a monitored region, selecting at least two-frame images with an overlapped region from the multiple-frame images, performing comparison between the selected images by image compensation method or feature matching method, and outputting monitoring information containing a mobile target which moves relative to a static target based on the result of comparison, wherein the position of the mobile target has an attribute of indefinite change, the mobile target in the monitored region can be recognized precisely during movement of the mobile robot, and monitoring information about the mobile target can be generated to prompt correspondingly, thereby safety of the monitored region can be effectively ensured.