Electromagnetics is one of the important contents of physics courses in engineering colleges. Many abstract formulas such as Maxwell’s equations in electromagnetism are difficult for freshmen who are new to college physics. In this paper, the core content of “field” is revolved around and the entire electromagnetism content runs through via the clue of “experimental laws → properties of the field → interaction between the field and matter”. It is discussed how to realize the sublimation from point charge to field, and finally reach a unified theory of the field. Taking a moving point charge with variable speed as an example, it is illustrated how the solution of Maxwell’s equations under the field description exceeds Gauss theorem of electrostatic field. Guided by problems, students are directed to gradually understand the core content of electromagnetism by exploration and thinking.