In this work, spindle-like micrometre SrMoO$_4$:Ln$^{3+}$ (Tb, Eu) phosphors have been synthesized and designed as a fluorescent sensor for Fe$^{3+}$ ions assay. The structural information, morphologies and luminescence properties of the samples were characterized by X-ray diffraction, Fourier-transform infrared, Raman analysis, field-emission scanning electronmicroscopy and photoluminescence patterns. Furthermore, Fe$^{3+}$ ions could be immediately detected using fluorescence quenching methods, and this method shows excellent and satisfying sensitivity. This facile method could be extended to environmental and biological applications.