In order to achieve the reduction and recycling, anaerobic digestion of sludge is the most widely used treatment method in the world. Volatile fatty acids(VFAs), an important product in acidification stage, could be used to remove nitrogen and phosphorous from wastewater as carbon sources, and to synthesize valuable products, such as bioplastics polyhydroxyalkanoates(PHAs). Metabolic mechanism and microbial mechanism of acidogenesis process during sludge anaerobic fermentation were introduced. The research results of anaerobic fermentation of sludge in recent years were reviewed systematically. The effects of substrate species, pretreatment technology, pH value and fermentative temperature on the acid production by anaerobic fermentation of sludge and its research progress were discussed. Temperature and pH value obtained more attention and it was summarized and compared the acid production by sludge fermentation under different substrate types, temperature, acid and alkaline conditions could affect the production and type distribution, while sludge pretreatment technology tended to increase the production of acid, but had little effect on the type distribution. The application of anaerobic fermentation in the synthesis of PHAs, bioenergy, nitrogen and phosphorus removal from sewage was also introduced. Finally, due to the different organic compoments of the substrate, the acidification efficiency would be different. Meanwhile, controlling the substrate type, pH value and temperature would not only affect the acid production, but also affect the acid production type. Therefore, the research directions of further study were given as the reasons for differences in acidification efficiency of different substrates, directional fermentation of sludge to produce acid, and achieving the regulation of various acids proportions in total VFAs.