Abstract: The electric field and the magnetic field have different forms but the same essence. For different inertial systems，they can achieve mutual transformation，which follow the fundamental principles of mechanics. With the aid of special relativity and basic law of mechanics，this paper attempts to analyze the mutual transformation process and forms of the electric field and magnetic field in a more intuitive way， thus the essence of the electric field and magnetic field coming from the same material can be understood more deeply.

Key words: unity of electric field and magnetic field, relativistic transformation, covariance of physical quantities