The invention relates to a synthesis method and application of a ratiometric fluorescent molecular probe for simultaneously detecting a fluorine ion and a sulfite radical. The ratiometric fluorescent molecular probe adopts a 2-(2-hydroxyphenyl)benzothiazole derivative as a matrix structure, and detects the fluorine ion and the sulfite radical based on excited-state intramolecular proton transfer (ESIPT) and intramolecular charge transfer (ICT) mechanisms, respectively. The probe has a maximum emission wavelength of 498 nm in an acetonitrile solution with a concentration of 80%, when the fluorine ion is added, the fluorescence spectrum of the probe has a red shift of 136 nm; and when the sulfite radical is added, the fluorescence spectrum of the probe has a blue shift of 127 nm. The differentiated detection of the two ions can be realized by the fact that the fluorescence spectrum of the probe has an obvious red shift or blue shift after the fluorine ion or sulfite radical is added, respectively, showing different fluorescence response signals. The inventive fluorescent probe has the advantages of simple operation, mild reaction conditions, easy purification, high synthesis yield, good selectivity, high sensitivity and stable optical performances. At the same time, the design and synthesis of the fluorescent probe provide an important foundation for development of multi-functional fluorescent probes in the future.