The characteristics of this book take the shrinkage of the electric field in the motional direction as the fundamental theory to unify all principles in classic electromagnetics. The book points out that electromagnetic induction, electromagnetic wave, displacement current and Lorentz force are superficial phenomena and the intrinsic reasons for the generation of these phenomena are a result of the motion of electric field. It indicates that the magnetic field is actually the electric field that is able to shrink in motion. The magnetic field is not a substance that independently exists and its energy comes from the increase of strength of the electric field as a result of shrinkage of the electric field in motion. This book deduces that the constancy of the velocity of light is the root of magnetism and the velocity of light of 299792458m/s is an ideal velocity. The author believes that readers will have a deep understanding of the nature of electromagnetic through the fundamental theory discussed in this book.