We developed a photothermal sensing system based on the photothermal effect of the Tetramethyl benzidine/Hydrogen Peroxide (TMB-H2O2) colorimetric system mediated by Fe3O4 nanoparticles. The ochratoxin A (OTA) aptamer can bind OTA particles with high affinity and specificity. The charge transfer complex of the single electron oxidation product of TMB (oxidized TMB) acts as a photothermal probe to convert the recognition signal into heat, allowing sensitive and quantitative photothermal detection using only a thermometer. In the proposed sensing system, the temperature change varied with the concentration of OTA. The aptamer-based sensing system provides a wide detection range for OTA from 0.25 nM to 2500 μM. We provide a new approach for the detection of mycotoxins because of the advantages of the simple readout method, low sample consumption, and lower detection cost.