Abstract: The abstract concepts of X-ray diffraction and reciprocal lattice are difficult to learn.The theoretical basis of X-ray diffraction analysis is reciprocal lattice，and the construction of reciprocal lattice is based on X-ray diffraction.The two concepts are based on each other.However，X-ray diffraction and reciprocal lattice are explained separately in most textbooks，which brings difficulties for learning and understanding.In this paper，the principle of X-ray diffraction is introduced systematically.Then the reciprocal lattice based on X-ray diffraction，X-ray diffraction and reciprocal lattice are explained in a unified way in order to make learning and understanding easier.

Key words: X-ray diffraction, reciprocal lattice, lattice extinction