A new aromatic diamine monomer which contains phenolphthalein structure, isopropyl and methyl groups was prepared by thymolphthalein and 4-fluoronitrobenzene through aromatic nucleophilic substitution and oxidation-reduction reaction. Furthermore, a series of high soluble polyimides (PI-1, PI-2 and PI-3) were prepared through one step high temperature condensation polymerization with the new diamine monomer and three commoditized aromatic dianhydrides (4,4’-oxydiphthalic anhydride, 4,4’-arbonyldiphthalic anhydride and 4,4’-biphthalic anhydride). The obtained polyimides were not only dissolved in high boiling point solvents, such as NMP、 DMAc、DMF and DMSO at room temperature, but also could dissolve in low boiling point solvents, such as CHCl3, CH2Cl2 and THF. The polyimides could be easily prepared for film materials by solution casting method. The prepared polyimide films have better optical transparency. Their cut-off wavelengths were between 324 nm to 365 nm, and their transmittance at the wavelength of 450 nm was in the range of 56%~78%. Their glass transition temperatures were between 265 ℃ and 288 ℃, and their temperatures of 10% weight loss were both beyond 430 ℃ in N2 and O2. Meanwhile, these polyimides exhibited good mechanical and dielectric properties. Their tensile strength and elongation at break ranged from 77 MPa to 95 MPa and 9.1% to 13%, and their dielectric constants were in the range of 2.79~3.01 at 1MHz.