1. A motion detection method, comprising: capturing a current frame; generating a current luma frame according to the current frame; generating a foreground binary image according to the current luma frame, a background luma frame and a sensitivity; and updating the background luma frame according to an updating frequency and the sensitivity; wherein the step of generating the foreground binary image comprises: generating an original binary image according to the background luma frame and the sensitivity, the original binary image comprising a plurality of original changing points with a first binary value and a plurality of original non-changing points with a second binary value; setting the original changing points neighboring to the original non-changing points to the second binary value to generate a noise-reduced binary image, the noise-reduced binary image comprising a plurality of noise-reduced changing points with the first binary value and a plurality of noise-reduced non-changing points with the second binary value; and setting the noise-reduced non-changing points neighboring to the noise-reduced changing points to the first binary value to generate the foreground binary image.