A thin-wall tube connecting assembly includes a connecting bar, and a tightening nut connected to the connecting bar through threads. The connecting bar is in a hollow tube-shape. One end of the connecting bar includes an expansion cone having an exterior diameter that is larger than that of other parts of the connecting bar. Another end of the connecting bar includes exterior threads. An elastic tube sits between the expansion cone of the connecting bar and a tightening nut. A rigid ring wraps around the exterior of the elastic tube, which shares a same axis with the tightening nut. A tube to be connected has an interior diameter that is equal to or larger than the exterior diameter of the expansion cone of the connecting bar. The rigid ring has an interior diameter that is equal to or larger than an exterior diameter of the tube to be connected.