An online monitoring device was designed and accomplished to realize monitoring of ferromagnetic abrasive particles in oil in real time.Based on the characteristics of GMR magnetic sensor which can effectively detect weak magnetic field,GMR magnetic sensor was used to monitor the residual magnetic field of a single iron abrasive after being magnetized.The abrasive particle information was obtained from the magnetic field signal output by the GMR magnetic sensor in the form of “analog online”.By optimizing the device and experimental parameters,the influence of temperature and moving speed of particles on the output of test system was studied,and the detection ability of the sensor and the consistency of the detection results were discussed.The results show that under the condition of the temperature within 95 ℃and the moving speed of abrasive particles within 5 cm/s,the output of the GMR magnetic sensor is almost unaffected by temperature and moving speed of abrasive particles.When the speed of magnetic abrasive particles is constant,the output of GMR magnetic sensor has good consistency for different sizes of magnetic abrasive particles.The experiment preliminarily shows that GMR magnetic sensor can be used for online monitoring of ferromagnetic abrasive particles in oil,and can effectively detect ferromagnetic abrasive particles with sizes above 75 μm.