A photoresist removal method is provided. The photoresist removal method includes analyzing the process status of each of a number of semiconductor substrate models undergoing a tested plasma ash process by a residue gas analyzer. The tested plasma ash processes for the semiconductor substrate models utilize a plurality of tested recipes. The photoresist removal method further includes selecting one of the tested recipes as a process recipe based on the analysis results from the residue gas analyzer and at least one expected performance criterion. In addition, the photoresist removal method includes performing a plasma ash process on a semiconductor substrate according to the process recipe to remove a photoresist layer from the semiconductor substrate.