Magnetic mesoporous nanoparticles of Fe₃O₄@mSiO₂ were prepared through co-precipitation and tetraethoxysilane hydrolysis at 60 °C and pH 10 with hexadecyl trimethyl ammonium bromide as the template. Hydroxypropyl β-cyclodextrin was modified onto Fe₃O₄@mSiO₂ under sodium citrate and N₂ protection via an ultrasound process to increase the catalytic efficiency. The prepared nanoparticles were characterized by Fourier transform infrared spectrometry, X-ray diffraction, scanning electron microscopy and a peroxidase-like activity assay. The results showed that the prepared nanoparticles have magnetic properties, peroxidase-like activity and a loose spherical cluster structure. Compared with Fe₃O₄@mSiO₂ nanoparticles, Fe₃O₄@mSiO₂@HP-β-CD nanoparticles exhibit higher catalytic ability toward both H₂O₂ and 3,3′,5,5′-tetramethylbenzidine. Fe₃O₄@mSiO₂@HP-β-CD nanoparticles can catalyse β-estradiol (β-E₂) oxidation in the presence of H₂O₂ and be used as a colourimetric sensor for indirect detection of β-E₂. A good linear relationship was obtained from 0.8 μM to 16 μM. The limit of detection of the proposed method was 0.2 μM. The visual method was successfully used in the analysis of β-E₂ in commercial tablets and animal feeds, with recovery ranging from 92.6% to 110%.