The invention discloses a black hair dye and a hair dyeing method. The black hair dye is a two-agent hair dye taking pyrogallic acid and ferrous sulfate as main components. The hair dyeing method comprises the following steps: spreading a first agent on hairs; uniformly combing; standing for 15-20 minutes; scraping residual first agent on the surface of the hairs; directly spreading a second agent on the hairs without washing; uniformly combing; standing for 5-8 minutes; washing the hair dye of the hairs with clean water; and washing the hairs. The hair dye has the advantages of no aniline dye intermediate having anaphylactic reaction on human body, strong dyeing power, saturated coloring matter, durable dyeing effect, less possibility of fade, simple operation and no time consumption.