The photoluminescence properties of carbon quantum dots depend on their size and the properties of surface functional groups. The N-doped carbon dots (using small molecular ethylenediamine) with high quantum yield and excellent dispersibility were synthesized by one-step hydrothermal method with biomass tar that was generated in the reductive smelting process as a precursor. Rapid and accurate Fe3+ detection based on the selective fluorescence quenching effect of N-doped carbon quantum dots was achieved. The results showed that the as-synthesized N-doped carbon quantum dots were regular spherical, uniform in size with an average particle size of 2. 64 nm with a quantum yield of 26. 1%, and the crystal lattice spacing was 0. 25 nm, corresponding to the (100) facet of graphitic carbon structure. The functional groups on the surface of N-doped carbon quantum dots could interact with Fe3+ to form complex compound by coordination, leading to the fluorescence quenching effect. Fluorescence emission ratios kept a linear relationship with the concentrations of Fe3+ in the range of 0. 23-600 mu mol/ L with the detection limit of 230 nmol/L.