An auxiliary device used for accurately scribing special tubing is provided with a fixing plate and an L-shaped guide plate perpendicularly mounted at one end of the fixing plate. One side of the fixing plate is provided with numerical graduations, the middle of the fixing plate is provided with a rectangular groove, a fixing bolt arranged in a mounting hole of the L-shaped guide plate and a fixing nut mounted on the fixing bolt are penetratingly arranged in the rectangular groove, and the L-shaped guide plate is fixed on the fixing plate. The auxiliary device is simple in structure and convenient to use, only one operator can scribe lines by sliding the free ends of the guide plate and the fixing plate on the tubing in a contact manner, thus, labor intensity of the scribing operator is greatly reduced, and investment in labor resources is reduced. Besides, during sliding, the size of the free end of the fixing plate is kept constant by the fixing bolt and the fixing nut, and scribing quality is effectively guaranteed. Meanwhile, production cost is greatly reduced and production efficiency is improved.