Abstract: The colorless polyimide (PI) used for the flexible display field is required to have high transparency and low thermal expansion coefficient C_TE. However, the C_TE value of commercial colorless PI film is above 40×10⁻⁶/℃. In the study, nanofiber self-filled PI film was prepared by embedding the PI nanofiber-net in thermoplastic fluorinated polyimide solution (PI-F). The nanofiber-net self-filled PI film demonstrates high transparency retention, much C_TE and excellent tensile property. The results show that the fiber of nanosize avoids light scattering, therefore the high transparency of the film is retained. When the PI fiber mass fraction is 10%, the transparency of the film is 80.5%. In comparison to the pure PI-F film, the C_TE of composite film is reduced by 40.3% and reaches 28.3×10⁻⁶/℃; The tensile strength and the tensile modulus of the composite film are increased by 132.9% and 89.5%, respectively and are 107.6 MPa and 1152.2 MPa correspondingly.