The invention discloses an ultramolecular gelator MP5 based on naphthalimide functional column 5 arene. The gelator can form organic ultramolecular gel MP5-G with yellow aggregation induced fluorescence in a cyclohexanol solution under the action of pi-pi; the organic ultramolecular gel can be used for detecting Fe<3+> with high selectivity and super sensitivity: when ions such as Fe<3+>, Hg<2+>,Ag<+>, Ca<2+>, Cu<2+>, Co<2+>, Ni<2+>, Cd<2+>, Pb<2+>, Zn<2+>, Cr<3+> and Mg<2+> are added into the MP5-G, fluorescence is quenched only by the Fe<3+>, and stable metal gel MP5-FeG is formed. When L-Cys is added into the fluorescence-quenched metal gel, the L-Cys and the Fe<3+> are complexed to recover the pi-pi action again among the MP5-G, so that aggregation induced fluorescence appears again,and continuous reversible ultrasensitive detection of the Fe<3+> and the L-Cys is realized.