The present disclosure provides a top-gate self-aligned metal oxide semiconductor TFT and a manufacturing method thereof. By providing a light-shielding layer below an active layer to protect the active layer from light irradiation and prevent the TFT from generating a negative threshold voltage drift phenomenon. Further, by connecting the light-shielding layer to the source, a stable voltage is generated on the light-shielding layer to avoid the floating gate effect, so as to improve the working stability of the TFT effectively. The top-gate self-aligned metal oxide semiconductor TFT produced by the method of the present disclosure does not generate negative threshold voltage drift phenomenon and floating gate effect, resulting in good working stability.