A complementary paper tube adapted to fill a remaining space on a cargo pallet includes sidewall portions, a straight auxiliary tearing line and circular auxiliary tearing lines. The side wall portions are disposed adjacently to each other. The straight auxiliary tearing line surrounds the sidewall portions. The circular auxiliary tearing lines are disposed on the sidewall portions and respectively connected to the straight auxiliary tearing line. A portion of the area surrounded by the circular auxiliary lines is adapted to be torn away, so as to form circular openings. The complementary paper tube is adapted to be torn into an upper tube and a lower tube. In addition, a flat plate adapted to form the complementary paper tube is also mentioned.