In order to improve the piezoelectric properties of polyacrylonitrile (PAN) fiber membranes, ionic liquid 1- allyl- 3- butylimidazole tetrafluoroborate (IL) was doped into PAN, and PAN/IL composite nanofiber membranes were prepared by electrospinning technology. The surface morphology, conformation, piezoelectric properties dielectric properties, mechanical properties and sensor sensitivity of PAN/IL composite fiber membranes were characterized by scanning electron microscope, Fourier transform infrared spectroscopy, X- ray diffraction,broadband dielectric and impedance spectrometer, stretching machine, LCR tester and other methods. The research results show that with the addition of an appropriate amount of IL, the PAN/IL composite fiber membrane has a good morphology without beading, and the fiber arrangement has a certain orientation. Doping IL into PAN can increase the content of the planar zigzag conformation of the composite fiber membrane, and its value can reach up to 93.08% , which is beneficial to the improvement of the piezoelectric properties of the PAN composite fiber membranes. After adding ionic liquid (IL), the dielectric constant of the composite fiber membrane increases from 3.40 to 6.14, the dielectric loss decreases from 0.05 to 0.03, the stress can reach 4.27 MPa and the sensitivity of the prepared flexible sensor can reach 0.56 kPa^-1.