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.