The invention discloses a Fe<3+> detection porous conjugated polymer and a preparation method and application thereof. A structural unit of the polymer is as shown in the specification, wherein R' refers to one of carbomethoxy, carbethoxy, tert-butoxycarbonyl, cyano, nitro and -COONa arbitrarily. By MBH-type reaction synthesis, mild reaction conditions, environment friendliness and freeness of metal catalyst residues are realized; since Fe<3+> ions have an evident quenching effect on fluorescence of a tetrahydrofuran solution or an aqueous solution of the synthesized porous conjugated polymer, the porous conjugated polymer can be used for detection of Fe<3+> ions in an aqueous phase or an organic phase and is high in Fe<3+> ion selectivity and low in Fe<3+> ion detection limit.