The invention provides a separating unit for separating a sheet material along a prefabricated weak line. The separating unit has the width direction and comprises at least a first roll and at least asecond roll, each first roll comprises a rotating axis extending in the width direction and the sheet width, each second roll comprises a rotating axis parallel to the rotating axis of the first rolland the sheet width in the width direction, and certain distance is formed between each second roll and the corresponding first roll and extends in the direction vertical to the width direction. Thefirst rolls and the second rolls each comprise at least one protrusive element extending vertically from the axis, and each protrusive element has the maximal width in the width direction, the maximalradial direction extending size from the rotating axis, the inner part adjacent to the rotating axis and the outer part away from the rotating axis. The outer parts of the protrusive elements on thefirst rolls and the outer parts of the protrusive elements on the second rolls are staggered so that the radial length parts of the outer parts of the protrusive elements on the first rolls and the radial length parts of the outer parts of the protrusive elements on the second rolls can overlap partially to form corrugate channels. The radial overlapping length is 2-40 mm, or 2-20 mm, or 3-12 mm or 4-10mm.