A new aromatic diamine monomer with phenolphthalein and methyl groups was prepared by o-cresolphthalein and 2-chloro-5-nitrotrifluorotoluene through aromatic nucleophilic substitution and oxidation-reduction reaction. Then a series of copolymerized polyimides which contains phenolphthalein structure and bulky groups were prepared by diamine monomer (2d) and commercial 2,6-diaminotoluene (TDA) with ODPA in different proportions through using a variety of monomer copolymerization methods, the structure and performance of copolymerized polyimides were tested. The series copolymerized polyimides had excellent solubility, they were not only dissolved in high boiling point solvents, such as NMP、DMAc、DMF and DMSO, but also could dissolve in low boiling point solvents, such as CHCl₃, CH₂Cl₂ and THF at room temperature. Their glass transition temperatures were between 275~314 ℃, and their temperatures of 10% weight loss were in the range of 477~507 ℃ and 477~490 ℃, respectively. Meanwhile, these polyimides had low dielectric constants and good thermal properties. The dielectric constants of the polymer films ranged from 2.69 to 2.92 at 1MHz. The tensile strength, elastic modulus and elongation at break of copolyimides ranged from 80 MPa to 92 MPa, 1.7 GPa to 2.0 GPa, and 9.2% to 13.0%, respectively.