The invention discloses a ferroferric oxide nano-composite particle and a preparation method and applications thereof. The composite particle is a ferroferric oxide nano-composite particle with the particle size of 10-30nm, and 1,4-dihydroxy anthraquinone and fluorenylmethoxycarbonyl are modified on the surface of the ferroferric oxide nano-composite particle. The preparation method comprises the steps of: firstly, carrying out a hydrothermal method on ferric trichloride hexahydrate, sodium acetate, 1,6-hexanediamine and ethanediol to obtain the ferroferric oxide nano-composite particle, then producing 1,4-dihydroxy anthraquinone, chloroacetyl chloride, and N,N-dimethylformamide into dyes, then adding the ferroferric oxide nano-composite particle, sodium carbonate and the dyes into acetonitrile to carry out a reflux reaction, carrying out solid-liquid separation, washing and drying on the obtained reaction liquor to obtain an intermediate product, then dispersing the intermediate product into the N,N-dimethylformamide, adding dispersion liquid into the fluorenylmethoxycarbonyl, stirring for 2 hours, and carrying out solid-liquid separation, washing and drying to obtain a target product. The ferroferric oxide nano-composite particle can be used for fast detecting four heavy metal ions, namely copper, zinc, cadmium and mercury in an aqueous solution.