Abstract: The disclosure discloses a preparation method and application of an external carbon source for denitrification from lake algae, and belongs to the technical field of solid organic waste treatment and utilization and water treatment. According to the disclosure, a combined process of struvite precipitation+calcium phosphate precipitation is used to recover nitrogen and phosphorus from an anaerobic fermentation liquor of the lake algae, and the treated anaerobic fermentation liquor of the lake algae can replace traditional commercial carbon sources as the external carbon source in a process of nitrogen removal. The treated anaerobic fermentation liquor of the lake algae cannot only provide the carbon source for a denitrification process, but also significantly improve nitrogen removal capacity of sewage compared with the commercial carbon sources.