Abstract: With the increase of public attention on food safety, it is important to establish a sensitive and rapid detection method for progesterone（pregn-4-ene-3,20-dione, P₄） in animal food products. A magnetic relaxation switch（MRS） sensor based on aptamer functionalized Au@Fe₃O₄ nanoparticles（NPs） was developed for the detection of progesterone in milk. The test conditions were optimized to improve the sensitivity and stability of the method, and the feasibility of this method for the detection of progesterone in milk was also analyzed.The ratio of aptamers（Apt） to Au@Fe₃O₄ NPs, the incubation time of aptamers modification, the concentration of NaCl, the dilute ratio of Apt-Au@Fe₃O₄ and reaction time were optimized to be 3, 1.5 h, 0.02 mol/L, 400 and 15 min, respectively. Under the optimized conditions, the change of mono-component relaxation time(ΔT_{2 w}) showed a linear range from 1 nmol/L to 40 nmol/L and the limitation of detection（LOD） was 4.97 nmol/L. In addition, the proposed method also illustrated an excellent specificity. The method was also applied successfully for the detection of progesterone in milk samples. The MRS displayed linear ranges of detecting progesterone from 20 nmol/L to 100 nmol/L and LOD of 13.60 nmol/L in milk. The average recoveries of progesterone in milk were 99.21%～103.45%, with relative standard deviations of nomore than 9.02%. The results indicated that this sensor could be a potential tool for the rapid detection of progesterone in food.