With the implementation of stricter ammonia nitrogen wastewater discharge standards, the problem that the effluent of most urban sewage treatment plants in China is difficult to meet the standards is becoming increasingly prominent, in which the lack of carbon source becomes a limiting factor of denitrification. For low carbon nitrogen ratio wastewater, additional carbon source should be added to enhance denitrification. Based on the research results on denitrification and external carbon sources, this paper summarizes the traditional carbon sources mainly composed of small molecular organics and carbohydrates, the new type of solid carbon source mainly based on natural cellulose materials, synthetic polymers, and skeleton composite slow-release carbon sources, and the new type of liquid carbon source mainly based on industrial wastewater, sludge and kitchen waste hydrolysate, and analyzes the problems and challenges in the current research and practical application of external carbon sources. The results show that the hybrid carbon sources based on traditional carbon sources are more suitable for commercial promotion at present, while most of the new carbon sources still have various problems in practical application, but they are worth further study due to their low cost, high environmental protection and wide application.