The invention relates to a phosphorus-doped fluorescent carbon point with a high fluorescent quantum yield as well as a preparation method and application thereof. According to the technical scheme, purified carbon point solid powder is obtained by taking tannic acid and polyethylenimine (PEI), adding de-ionized water and phosphoric acid, then transferring the mixture into a hydrothermal reaction kettle, keeping the heat and reacting for 2h to 6h in a temperature range of 140 DEG C to 240 DEG C, naturally cooling at a room temprature to obtain a light brown nano carbon point crude product solution, and centrifuging, dialyzing, refining, freezing and drying. The carbon point provided by the invention can be used for cell imaging and a photodegradation treatment process of contained organic dyestuffs. The fluorescent quantum yield of the prepared fluorescent carbon point can reach 25.4 percent. After being stored in a dark place for 6 months, the carbon point still can emit bright blue fluorescent light under the illumination of a 365nm ultraviolet lamp and the carbon point has good stability at a room temperature.