Steel tube inner and outer flange joints are widely used in large-span transmission towers. Filling concrete in chord has become an effective method to improve the bearing capacity and the stress on steel tubes. In order to reveal the influence of filled-concrete on the mechanical properties of steel tube inner and outer flange joints under axial tensile load, two concrete-filled inner and outer flange joints (CFIOF) and two inner and outer flange joints (IOF) were designed for axial tensile tests. The failure mode, flange opening deformation, strain development of key parts and axial tension ratio β of outer and inner bolts were compared and analyzed. Then, a CFIOF finite element model was established and validated for parameter analysis to reveal the influence of flange thickness, bolt spacing, and concrete strength on β. The study found that the failure mode of CFIOF is basically the same as that of IOF, both of which are flange plate opening, accompanied by deformation of the bolt thread area. When the opening deformation is the same, the bearing capacity can be increased by more than 5%. Filling concrete can make the inner and outer bolts more uniformly under stress. As the thickness of the flange plate increases, β shows a trend of first increasing and then decreasing, gradually approaching one. With the increase of bolt spacing and concrete strength, β decreases gradually and tends to 1. The effect of concrete strength on β is the smallest.