The utility model relates to a light cylindrical steel template with an adjustable arc length. The light cylindrical steel template comprises a reference arc type steel plate and at least one extension arc type steel plate, wherein the reference arc type steel plate and the extension arc type steel plate are equal in height and thickness and are surrounded and spliced into a barrel through a bolt; reference surrounded-strakes which are equal to the reference arc type steel plate in the radian and the arc length are arranged on the outer wall of the reference arc type steel plate at intervals, and extension surrounded-strakes which are equal to the extension arc type steel plate in the radian and the arc length are arranged on the outer wall of the extension arc type steel plate at intervals; and the reference surrounded-strakes and the extension surrounded-strakes form an integral ring on the same horizontal plane. The reference arc type steel plate and the extension arc type steel plate after surrounding and splicing can be strapped and be closely fitted with a template body, so that the template safety in the pouring process is ensured, and the integrity of the template is increased. The template can be widely applied to construction of various concrete cylinders with different diameters.