A manufacturing method of a thin film transistor, a manufacturing method of an array substrate and an array substrate are provided. The manufacturing method of the thin film transistor comprises: forming an active layer, a source electrode and a drain electrode on a substrate by one patterning process, the active layer, the source electrode and the drain electrode being located in a same layer. The manufacturing method of the thin film transistor can effectively reduce the number of patterning processes, so as to enhance the capacity in mass production, and reduce the cost.