The invention provides an open seam detector for a cigarette filter rod. The open seam detector can detect the open seam length of external wrapping paper seams of cigarette filter rods with different specifications. The open seam detector consists of a base bracket, two torque motors, a track, two elastic claws, a button, measuring calipers and an electric control system, wherein the cross section of the base bracket is L-shaped; the two torque motors are connected to the base bracket; an interval between the two torque motors can be regulated by using a handle of an interval regulator; when an open seam is formed after the filter rod is twisted, the length of the maximum open seam can be measured visually or automatically; the power range of each torque motor is 15 to 45w; and the range of twisting speed is 0.02 to 0.2 second per turn. The open seam detector for the cigarette filter rod improves the working efficiency and the detection accuracy, can detect cigarette filter rods with external wrapping paper and with the different specifications, and is reasonable in structure, convenient to operate, and easy to maintain.