A new fluorene-based diamine with tera-methoxy groups, 9,9-bis(3,5-dimethoxy-4-(4-amino-phenoxy)phenyl)fluorene(3) was synthesized by three-step reaction. A series of soluble polyimides (PI 4a-4d) were prepared from the diamine monomer with four commercial aromatic dianhydrides via high temperature polycondensation. The structures of the monomer and polymers were confirmed by ¹H-NMR and FT-IR, and the properties of these copolymers were also studied by DSC, TGA, etc. It is found these polyimides have good solubility in NMP, DMAc, DMF, DMSO, CHCl₃, CH₂Cl₂, etc.The films prepared from their solution display good mechanical properties, and their tensile strength, elongation at break and elastic modulus are in the range of 62.1～75.6 MPa, 16.0%～20.7%, 1.8～2.6 GPa, respectively. These polyimides also show good thermal properties. Their glass-transition temperature ranges from 244 ℃ to 256 ℃ and the 10% mass loss temperature in air and nitrogen is higher than 430 ℃.