The invention discloses a fermentation method of using zero-valent iron to promote conditioned sludge to generate L-lactic acid. The fermentation method particularly includes: injecting organic waste into a fermentation reactor, and adding zero-valent iron for first-stage medium-temperature hydrolysis; adding the zero-valent iron into the reactor again for second-stage high-temperature anaerobic fermentation; adding alkaline liquid into the reactor, and regulating pH of a fermentation matrix to 7-11 for third-stage high-temperature alkalinity-adjusting anaerobic fermentation; taking the fermentation matrix for sludge-water separation, and centrifuging to obtain supernate which is the L-lactic acid. Diversity of microbes in remaining sludge, the characteristic that the organic waste is high in organic matter content and biocatalysis action of the zero-valent iron are utilized comprehensively, and the L-lactic acid high in added value is generated, so that the method is a waste recycling method which is efficient and low in energy consumption.