The invention provides a motion detection method, a device and an intelligent monitor system. A background difference image is obtained according to the prior input image and the prior background image respectively and an interframe difference image is obtained according to the prior input image and the previous frame input image thereof. Because the background different image includes foregroundinformation in the prior input image differentiated with the background and the interframe different image includes motion information in the prior input image, the invention combines the two part ofinformation to obtain motion foreground information.