Abstract: A novel ratio colorimetric sensor based on magnetic nanoparticles (Fe₃O₄@Au) and silver nanoparticles (Ag NPs) was constructed for the detection of Ochratoxin A (OTA) by complementary DNA base pairing. The OTA aptamer coupled Fe₃O₄@Au was served as reference signal, and complementary DNA (cDNA) coupled Ag NPs was served as the response signal. A ratio nanoprobe (Fe₃O₄@Au-Ag) with two UV absorption peaks was prepared by regulating the ratio and DNA base complementarity. In absence of OTA, the absorbance peak ratio (A₄₀₀/A₅₈₀) of the ratio colorimetric sensor could maintain a constant value. The detection conditions (time, pH, and temperature) were optimized, and under optimized conditions, the sensor showed a good linear relationship to OTA detection, with a linear range of 0.01 ng·mL⁻¹ to 1000 ng·mL⁻¹ and a detection limit of 0.033 ng·mL⁻¹. The sensor had good selectivity, reproducibility and high sensitivity, which was successfully applied to the detection of OTA in pea powder with a recovery rate of 93.9%~96.0%.