A device and a method for detecting cigarette fly ash by a gray-scale difference based on machine vision (MV) are provided. A manipulator holds a cigarette as a detection sample to simulate a human smoking action, and multiple groups of cameras track a simulated smoking process of the detection sample synchronously in real time. It is determined whether fly ash appears based on a gray-scale difference of burning ash columns, produced without being subjected to flicking, in acquired images. A fly ash area of the burning ash columns of the cigarette is calculated by an area with the gray-scale difference of the burning ash columns of the cigarette in two sequential images of the burning ash columns of the cigarette, and a fly ash amount is further determined. The detection of the amount of fine fly ash is converted into the detection of the gray-scale difference.