Finite element analysis and scanning electron microscope were conducted to investigate the bulging deformation and fracture of tubes in double-sided hydroforming. The effect of the external pressure imposed on the tube, which determines the magnitude of superimposed hydrostatic pressure, on the stress state, yield locus, fracture surface formation, and fracture strain was evaluated. The simulation results revealed that sufficiently high external pressure can change the stress state of the tube in double-sided hydroforming from an in-plane biaxial tensile stress state to a three-dimensional stress state, and it can increase its hydrostatic pressure in a superimposed manner. Moreover, double-sided free bulging and corner filling experiments were conducted on 5A02 aluminum alloy and 2A12 aluminum alloy tubes. It was found that the external pressure has a significant impact on the fracture behavior of these tubes. The increasing external pressure could change the type, number, size, and proportion of the dimples on the fractured surface, and transform the fracture mode from a void accumulation fracture to a pure shear fracture, which significantly improves the fracture limit of the tubes. These results are significant for the consolidation of the theoretical and numerical simulation prediction of the superimposed hydrostatic pressure effect in the hydroforming process.