The invention relates to the field of processing technology equipment for aerospace products, in particular to an internal supporting positioning device for girth welding of flange plates of carrier rocket tanks. The internal supporting positioning device comprises a power mechanism, a circumferential overall rotating mechanism, a radial supporting-tightening mechanism and an axial pressing mechanism. One end of the circumferential overall rotating mechanism is coaxially and fixedly installed at one end of the power mechanism. The radial supporting-tightening mechanism is coaxially and fixedly installed at the other end of the circumferential overall rotating mechanism. The axial pressing mechanism is coaxially and fixedly installed at the other end of the radial supporting-tightening mechanism. External workpieces are fixed between the radial supporting-tightening mechanism and the axial pressing mechanism in the axial direction. The power mechanism drives the circumferential overall rotating mechanism to achieve rotating movement of the workpieces in the axis directions of the workpieces. The device can internally support and tighten the two welded workpieces simultaneously, coaxiality adjustment is completed while supporting and tightening are conducted, the welded workpieces can be clamped in the axial direction, and the device needs to have enough structural rigidity so that welding deformation of products can be borne and reduced.