An object detection system and method is applied to an active surveillance system for detecting at least one target object appeared in an area of interest. The object detection system includes a processor for defining the area of interest into at least a first and a second zone; a controller for assigning a first and a second weight to the first and the second zone, respectively, wherein the first weight is higher than the second weight; and a first image capturing device for capturing an image of the target object appeared in the first zone prior to capturing an image of the target object appeared in the second zone. With these arrangements, the detection of at least one target object appeared in more than one zone having a specific weight assigned thereto may be implemented with improved efficiency and accuracy in object selection and reduced interferences from environmental variables.