The present invention provides a method for detecting a specific object in an image to be detected, including: a feature extraction step for extracting an image feature of the image to be detected; and a detection step for detecting detection windows with various sizes of the image to be detected according to the extracted image feature by using classifiers with various sizes corresponding to at least a part of the detection windows with various sizes, so as to determine the presence and location of a specific object in the image to be detected. The invention further provides an object detection device and a system including the device. The method, device and system for detecting a specific object in an image to be detected can improve the precision and increase the speed of the object detection.