In physics teaching, electromagnetism is one of the most difficult concepts for students to understand. This paper proposes a real time visualization method for 3-D magnetic field based on the augmented reality technology, which can not only visualize magnetic flux lines in real time, but also simulates the approximate sparse distribution of magnetic flux lines in space. An application utilizing this method is also presented. It permits leaners to freely and interactively move the magnets in 3-D space and to observe the magnetic flux lines in real time. As a result, the proposed method visualizes the invisible factors in 3-D magnetic field, with which students will have real-life reference when studying electromagnetic.