Top-gate self-aligned a-IGZO TFT is suitable for driving OLED displays for its tiny parasitic capacitors. In order to improve the stability of TFT device under illumination condition， the driving TFT usually needs light shading layer. In this paper， the influence of the light shading layer， the selection of its signal connection， the thickness of the buffer layer and the size of the light shading layer were explored. The results showed that： （1） the light shading layer could effectively reduce the negative bias caused by the light of TFT， （2） the light shading layer connecting signal was more stable than the floating one， and the TFT whose shading layer was connected to the source electrode was more easily saturated， which could be helpful to carry out OLED， （3） buffer layer thickness of 400 μm was more favorable for device performance， （4） the light shading layer should cover IGZO， and its size should be as large as possible， which could effectively improve the device reliability.