Abstract: Photosensitive polyimide (PSPI), as a core material in the microelectronics industry, exhibits excellent thermal stability, mechanical properties, and dielectric characteristics. However, its insufficient photosensitivity and retention rate are critical issues limiting its application in photoresists. Therefore, the development of polyimide photoresists with high sensitivity and high retention rate holds significant research importance. Using gallic acid derivatives, namely 3,4,5-trihydroxy-N-(2-hydroxyethyl) benzamide (GEA) and methanol as raw materials, a high-performance polyamic ester polymer (OGMFPAE) was synthesized via ring-opening reaction with the dianhydride monomer 4,4'-oxydiphthalic anhydride (ODPA), followed by polymerization with 2,2-bis(3-amino-4-hydroxyphenyl) hexafluoropropane (6FAP). The structure and properties of the polymer were characterized using Fourier-transform infrared spectroscopy (FT-IR), proton nuclear magnetic resonance spectroscopy (¹H-NMR), ultraviolet-visible absorption spectroscopy (UV-Vis), and gel permeation chromatography (GPC). The characterization results confirmed the successful synthesis of the polymer, and OGMFPAE exhibited excellent optical transmittance at 365, 405 nm, and 436 nm. Furthermore, the lithographic performance of the photoresist formulated with the OGMFPAE polymer was evaluated using a proximity exposure system. Results demonstrated that the prepared PSPI photoresist possesses high sensitivity (E_th < 50 mJ/cm²) at a film thickness of 2 μm, with a developed film retention rate exceeding 97% and a cured film retention rate greater than 90%. Even at an 11.5 µm film thickness, the photoresist maintains good photosensitivity, achieving a development retention rate higher than 90% and a curing retention rate higher than 75%. Well-defined photolithographic patterns with clear line profiles and high resolution were obtained, indicating promising application potential in organic light-emitting diode (OLED) displays, micro-electro-mechanical system (MEMS), and chip packaging.