1. A motion detection method, comprising: recording an input video having a plurality of image frames, with each image frame having a plurality of windows of interest (WOIs); extracting a reference frame from the input video in every reference updating cycle; extracting a target frame from the input video and determining whether an event occurs in each of the WOIs by comparing the target frame with the reference frame in every checking cycle; and comparing the number of WOIs in which the event occurs in every observation cycle with a threshold to determine whether the input video contains a motion during the observation cycle; wherein the observation cycle is greater than the checking cycle, and the checking cycle is greater than or equal to the reference updating cycle.