In this study, a novel approach and the related equipment of coaxial electrospinning have been developed to fabricate a new ultrafine polysulfone amide/polyurethane coaxial fibers at nanoscale, with the polysulfone amide as the core and the polyurethane as the shell of the blended fibers. As the co-spinneret has effects on the structure and properties of the spun fiber, three types of co-spinnerets with different diameters were designed to investigate its effects on the fabricated fibers in this research. Three series of polysulfone amide/polyurethane coaxial fibers were spun using the self-developed coaxial electrospinning equipment, and these fibers were characterized systematically using scanning electron microscope, transmission electron microscope, X-ray diffraction, differential scanning calorimeter and thermogravimetric. High-speed photography was used to digitalize the image of the tailor cone and jet motion of polymer fluid during the spinning process, which provides a detailed description of the electrospinning for the further theoretical analysis. The three-dimensional electric field simulation was also carried out to model the differences of electric field. Our experimental results show that the mechanical and thermal properties of the core-shell fibers fabricated in this research have been improved in the comparison with the fibers spun using the conventional single-needle electrospinning method. The composite fibers have the core-shell structure, so that it can combine the excellent thermal properties of the polysulfone amide and the excellent mechanical properties of the polyurethane. The newly developed polysulfone amide/polyurethane fiber could be used in the field of industrial textiles; it has the potential applications for the development of high-performance apparels in the future.