In four mask process, it is extremely critical to control the photoresist remain of halftone zone for the morphology and electrical properties of TFT. Therefore, this study focused on the key factors of photoresist remain in four mask process based on oxide-TFT technology. Firstly, the optimal conditions of soft-bake temperature, vacuum drying time and developing time are obtained via full factorial experiment. Furthermore, the ideal quantity of exposure is evaluated through single factor test. The results show that the experiment design could effectively optimize the remain and uniformity of photoresist, so that the ideal experiment conditions aere obtained. When the soft bake temperature, vacuum drying time, developing time and exposure are 115 ℃, 10 s, 52 s and 67 mJ/cm2, respectively, the PR remain and CD are 0.51 μm and 11.17 μm, which exhibites good uniformity (<5%).