The goal of this book is to study the unified field theory; that is, to logically derive all physical laws from a few simple principles. This goal is also the common dream of great thinkers who all agree that nature is governed by several extremely simple mathematical rules, and pure thought can grasp reality. Einstein spent most of his life in pursuit of the unified field theory. Clifford algebra is a unification of real and complex numbers, quaternion and vector algebra, which accurately reflects the intrinsic properties of the space-time. Clifford algebra provides a unified, standard, elegant and open language and tool for numerous complex mathematical and physical theories. Clifford algebra is a unified language for science and engineering, which can be expected to complete a new big synthesis of entire scientific knowledge.Based on four basic principles and Clifford algebra, all basic physical equations were derived, the logical relations between equations were reconstructed, and the solutions of some typical equations were solved. Meanwhile, the concepts of space-time and quantum theory were reasonably explained, and some important new conclusions were obtained.