The invention relates to a preparation method of a fluorescent molecular probe for detecting sulfite ions through naked eyes and fluorescence ratio as well as the application of the fluorescent molecular probe in detecting sulfite ions. The fluorescent molecular probe is prepared by condensing 4-hydroxy naphthalimide serving as raw material and acetylpropionic acid in refluent methylene dichloride. The synthesis is simple and convenient, and the reaction conditions are mild. The probe molecule provided by the invention has higher sensitivity, stable fluorescence performance, higher synthesis yield and good selectivity, and furthermore, the response range of the probe is 0-1500 mu m, the detection limit is 6 mu m; the detection range is wide, the lower detection limit is low, and the probe is suitable for naked eye detection. Meanwhile, the florescence-ratio detection is adopted to avoid errors caused by exciting light intensity, probe concentration and environment factors of an off-on type probe which detects ions only depending on the change of fluorescence intensity and the probe disclosed by the invention is not influenced by anions such as F<->, Cl<->, Br<->, I<->, HPO4<2->, SO4<2->, NO<3->, AcO<->, ClO<4->, N<3-> and HCO<3->. Even interfering ions exist, the probe has very good response to sulfite ions. Therefore, the fluorescent molecular probe has practical application value in the fields of biochemistry, environmental science and the like.