The content of magnetic iron is one of the basic analyses in ultra-low-grade magnetite exploration. In order to accurately determine the content of magnetic iron, quantitative separation of magnetic iron is very important. There is no standard method for the analysis of magnetic iron in iron ore, and the reproducibility of common artificial magnetic separation is poor due to the human factors and the difficulty of guaranteeing effective magnetic field intensity. A new magnetic separation device (electromagnetic separation device) was developed by using 50 mL burette, electromagnet and a three-phase asynchronous motor. Magnetic iron and nonmagnetic iron in ultra-low-grade magnetite was quantitatively separated by optimizing the working conditions. Determination of magnetic iron was proposed by combining the potassium bichromate titrimetric method with a magnetic separation technique. When the current was 2.5 A, the motion frequency of the magnetic tube was 40 times/min, and the time of magnetic separation was 5 min. The relative error was lower than 1.0% for the reference material. For the practical samples, the analysis results completed by the proposed method were in agreement with those determined by artificial magnetic separation with the RSD of the proposed method being lower than 1.0%, which was an improvement over the precision of artificial magnetic separation. For the new method, the magnetic field intensity was effectively controlled, the parameters for the separation of magnetic iron were quantized, and the measurement accuracy was improved for magnetic iron.