The invention discloses a europium-based coordination polymer nanosphere and a preparation method and application thereof. The preparation method of the europium-based coordination polymer nanosphere comprises the following steps of adding EuCl3.6H2O, H2BDC and polyvinylpyrrolidone to mixed solvent formed by mixing N, N-dimethylformamide and absolute ethyl alcohol, and conducting stirring till the materials are fully dissolved; then, transferring a mixed solution into a reaction still provided with a polytetrafluoroethylene inner container, and putting the reaction still in a drying oven at the temperature of 150 DEG C to leave the reaction still standing still for 12 hours; and then sequentially conducting cooling, centrifugalization, washing and vacuum drying, so that the europium-based coordination polymer nanospheres are obtained. A to-be-detected water body is added to fluorescent material suspension liquid prepared from the europium-based coordination polymer nanospheres; after being mixed uniformly, the materials are left standing still; and then a fluorescence spectrophotometer is adopted for detection, and the content of phosphate in the to-be-detected water body can be obtained. The europium-based coordination polymer nanosphere is stable in performance, not prone to photobleaching, good in water solubility, capable of effectively detecting the content of the phosphate in the water body, simple in synthesis method and low in raw material cost.