The work aims to explore experimental method of modifying surface of polyimide (PI) substrate with alkali solution based upon the characteristic of polyimide being dissolved in alkaline solution. Relationship between concentration, temperature, time and treatment conditions of NH3?H2O, NaOH and KOH solutions and modification effect of PI surface was studied to determine the optimal modification method. Polyimide substrates were treated with different concentration of NH3?H2O, NaOH and KOH solutions at room temperature or under hydrothermal conditions. Orthogonal experiments were performed to determine the optimal alkali concentration, temperature and treatment time for hydrothermal conditions. Change of contact angle before and after sample treatment was measured with contact angle meter, light transmittance was characterized with UV-visible spectrophotometer, and tensile strength and elongation at break were measured with smart electronic tensile tester. The best surface treatment conditions acquired by the experiments were: 25 μm thick PI substrates, 1.0 mol/L NH3?H2O solution, room temperature, for 30 minutes; 50 μm thick PI substrates, 0.05 mol/L NaOH, 120 ℃ hydrothermal temperature, for 90 minutes. For the 25 μm thick PI substrates, surface contact angle decreases to 45° after treatment with 1.0 mol/L NH3?H2O solution, indicating that NH3?H2O solution can effectively improve surface hydrophilicity of thin PI substrates. The hydrophilicity of KOH treated polyimide substrates increases significantly, but alkali concentration is not easy to control. NaOH hydrothermal surface modification method can better improve surface hydrophilicity of polyimide substrates, so as to improve interfacial adhesion between the substrate and the inorganic coating.