The invention belongs to the technical field of ship 
ballast water microalgae content detection, and particularly relates to an up-conversion nano fluorescent probe which realizes 
rapid detection of microalgae content through competitive emission. According to the method, a red up-conversion nano fluorescent probe is adopted, red 
luminescence of the fluorescent probe is highly overlapped with the maximum absorption peak of 
ballast water microalgae 
chlorophyll a under excitation of near-
infrared light, a relation curve of 
luminescence intensity and the content of 
chlorophyll a is established through competitive 
luminescence measurement, and the content of the 
chlorophyll a is determined according to the corresponding relation of the content of the 
chlorophyll a and the microalgae 
cell biomass. And establishing a relation model between the 
luminous intensity and the microalgae 
cell biomass so as to calculate the microalgae 
biomass in the 
ballast water sample. According to the present invention, the 
rapid detection of the microalgae biomass can be achieved, the operation is simple and convenient, the sensitivity is high, the safe 
long wavelength near-
infrared light is adopted as the excitation source, the 
light damage to the 
organism is reduced, the spontaneous 
background fluorescence of the microalgae cannot be caused, the detection sensitivity can be improved in the 
order of magnitude, and the method is suitable for promotion and application.