In practical applications, flexibility, lightweight, and high performance are the characteristics that polymer-based electromagnetic shielding materials should have. At present, it is still a great challenge to prepare polymer-based electromagnetic shielding materials with excellent conductivity, electromagnetic shielding properties, and mechanical properties. Therefore, in this work, single-walled carbon nanotubes/polyetherimide composite films are prepared by electrostatic spinning and vacuum-assisted filtration through using single-walled carbon nanotubes and polyetherimide as raw materials. By regulating the surface density of single-walled carbon nanotubes, the conductivity of the composite film can be enhanced to 1866 S/cm. For the electromagnetic shielding performance, the total electromagnetic shielding effectiveness of single-walled carbon nanotubes/polyetherimide composite film in Ku band (12–18 GHz) is in a range of 75.78–81.83 dB, which is higher than that of pure single-walled carbon nanotube film (65.19–69.81 dB). This is attributed to the formation of interfaces between the polyetherimide fibers and the single-walled carbon nanotubes, with more interfaces consuming more electromagnetic wave energy for a given range of single-walled carbon nanotube surface densities. For the mechanical properties, the maximum tensile strength and elongation at the break of the single-walled carbon nanotube/polyetherimide film are 1.13 and 1.5 times higher than those of the single-walled carbon nanotube film, with the values of 28.52 MPa and 7.91%, respectively. As the surface density of single-walled carbon nanotubes increases, the interaction between single-walled carbon nanotubes as well as the interaction between polyetherimide fibers and single-walled carbon nanotubes at the interface plays a role in enhancing the mechanical properties of the composite films. The single-walled carbon nanotube/polyetherimide composite films, as an excellent polymer-based electromagnetic shielding composite material, can be used in fields such as the protection of precision electronic instruments and wearable electronic devices.