Abstract: Biological treatment of wastewater produces lots of excess sludge, resulting in secondary pollution. Fermentation of excess sludge can produce short-chain fatty acids (SCFAs), which can be used as an additional carbon source of biological nutrient remover and is important to sludge reduction and wastewater treatment. Excess sludge can be treated by alkaline fermentation, surfactant fermentation, and co-fermentation with carbohydrate and so on to get fermented liquid rich in SCFAs without biological toxicity. Enhanced excess sludge fermentation in-situ produces SCFAs, 3 ~ 8 times more than the conventional fermentation technology. Enhanced fermentation of the excess sludge in situ and liquid reuse in right place can realize the comprehensive utilization of sludge in sewage plant and provide a new way of thinking for residual sludge treatment. This review introduces the technologies and the mechanism of enhanced excess sludge in-situ fermentation and summarizes the process parameters and application effect of fermentation liquid recycling.