The utility model discloses a detecting device for the diameter of a cigarette or a filter rod. The detecting device is characterized by comprising a cylindrical sample inserting platform and a CCD (Charge Coupled Device) imaging system, wherein the cylindrical sample inserting platform is used for placing the cigarette or the filter rod; a height adjusting plate is arranged at the bottom part of the sample inserting platform; the sample inserting platform is driven to rotate by a driving motor through a transmission part; the CCD imaging system consists of a point light source and a linear-array CCD device which are correspondingly arranged above the two sides of the sample inserting platform, so that the cigarette or the filter rod in the sample inserting platform is just positioned in an imaging view field of the CCD imaging system and is vertical to parallel imaging beams generated by the CCD imaging system. The detecting device disclosed by the utility model detects the diameter of the cigarette or the filter rod by utilizing the CCD imaging principle and the characteristic of high spatial resolution of the linear-array CCD device, has the advantage of high detecting accuracy, is applicable to detection for the diameter of the cigarette with the length of a filter tip being 40mm and the total length of the cigarette being less than 150mm and detection for the diameter of the filter rod with the length being 40-150mm, and has the characteristics of simple structure and convenience in use and carry.