In this work, molecularly imprinted polymer (MIP) particles were synthesized using a semicovalent method based on a specific thermally reversible bond, and these particles were used for the rapid detection of the azo dye acid orange II. The imprinted polymers-which were prepared via the covalent reaction of 3-(triethoxysilyl)propyl isocyanate with the template molecules-were attached to the surface of silica-coated magnetic nanoparticles, and a simple thermal reaction was then performed to remove the templates, leaving spaces with specific noncovalent bonds for target re-recognition. The conditions for the synthesis of the MIP were optimized during the polymerization experiments to improve the adsorption capacity and selectivity. The resulting polymers were characterized using Fourier transform infrared spectroscopy, scanning electron microscopy, and transmission electron microscopy to confirm their structure. The MIPs were subjected to an online solid-phase extraction and a magnetic molecularly imprinted solid-phase extraction procedure. For both methods, all samples were prepared with spiking levels of 5.0, 10.0, and 15.0 μg kg(-1) using high-performance liquid chromatography with UV/vis detection; after the preconcentration of 50-mL sample solutions, the enhancement factors reached 710 and 629. The limits of detection (signal-to-noise ratio of 3) were 9.83 and 17.41 ng L(-1), with relative standard deviations (n = 9) of 6.72% and 8.25%, respectively, for a 4.0 μg L(-1) standard template solution. These two methods were developed to quantify trace acid orange II contents in food and environmental samples; the recoveries ranged from 72 to 105% and from 70 to 94%, respectively.