The present disclosure provides a thin film transistor (TFT) and its manufacturing method. The method includes the following steps: sequentially depositing a buffer layer and a shielding layer on a substrate; forming an IGZO layer on and covering the shielding layer; processing the IGZO layer by annealing so that a portion of the IGZO layer is diffused by the buffer layer and has a conductor property; and forming a source and a drain so that the source and drain contact the portion of the IGZO layer. The present disclosure, through annealing the IGZO layer, the buffer layer makes portions of the IGZO layer contacting the source and the drain to have conductor property, thereby avoiding the prior art's complex process, simplifying the manufacturing of the IGZO TFT, and enhancing the production efficiency.