A thin film transistor substrate and its manufacturing method are provided. The thin film transistor substrate avoids semiconductor defects caused by acid corrosion of a metal oxide channel during an etching process of forming a source/drain electrode, and effectively prevents copper from diffusing downward into the metal oxide channel under high temperature conditions. Such configuration eliminates a need to additionally use a barrier material, reduces production costs, and prevents short-circuiting resulting from a residual barrier material.