The invention discloses a black oxidation method of a stainless steel cathode. The method comprises the steps of adding an electrolyte into an electrolytic tank, then adding three drops of nitrobenzene, uniformly stirring, then extending completely-cleaned parts into the tank, electrifying, and performing oxidation for 10 minutes. The problems that a chemical oxidation film layer is non-uniform in appearance and high in energy consumption and has heavy metal pollution can be solved by adopting the technical scheme disclosed by the invention; a film layer obtained by using the method is uniformly gray black to black in appearance, and is especially suitable for stainless steel parts for extinction required by optical instruments.