1. An object detection method, comprising: a. capturing an image, wherein the image comprises a plurality of sampling-windows; b. using a first-stage sub-classifier of a classifier to detect whether the sampling-windows contain an object therein; c. rotating the classifier at least one time by a predetermined rotation angle and performing the step b after each rotating, wherein when the object is detected within the sampling-windows, keep detecting whether the sampling-windows contain the object therein sequentially by a second-stage sub-classifier to N th -stage sub-classifier of the classifier with the same orientation, wherein N is a positive integer greater than 1; d. rotating the image at least one time by a predetermined image angle and performing the steps b and c after each rotating; and e. outputting the sampling-windows containing the object.