Using magnetized device for magnetizing coagulant research of rare earth coagulants this paper examined the effect that magnetic field intensity, the magnetization time, dosage, PH on the magnetic flocculation, and made trials to compare the unmagnetized water with magnetized water by controlling dosage and sludge volume. The research showed that when magnetic field strength for 76.3 mT, magnetization time for 5min, dosage for 3ml, PH of 9, could produce better coagulation effect of magnetized flocculation. In the condition of the same dosage, magnetic water effluent turbidity and COD obviously lower than the magnetized water, the volume of sewage sludge produced by magnetized water was obviously less than the unmagnetized water, and was about its a third, and the properties of magnetic water produced a more close-grained, and a faster settlement.