In order to study the structures and properties of polyimide (PI) nanofibers spun by “one-step method”, two kinds of PI nanofibers，of which the average fiber diameter ranges from 0.364 μm to 0.785 μm, were prepared through electrospinning technology by using thermoplastic polyimide powder as raw material, and a mixture of N, N-dimethylformamide (DMF), N, N-dimethylacetamide (DMAc) or Tween 80 as solvent. The structures and properties of PI nanofibers were characterized. The results are as follows. The surface of nanofiber has no microholes. The fiber diameter increases with the polymer concentration increased, and reduces first and then increases with the spinning voltage and tip-to-collector distance increased. In addition, the solvent has significant influence on the chemical structure and thermodynamic performance of PI nanofibers. The structure regularity, mechanical property (stress of 24.8 MPa) and thermal stability (thermal decomposing temperature of 535 ℃) of PI_DMF-DMAc are higher than those of PI_{Tween80-DMF-DMAc} nanofiber (the stress and thermal decomposing temperature are 16.5 MPa and 421 ℃, respectively).