In this paper, through analysis we points out definitely that the electric quadrupole moment of an arbitrary charge system is equivalent to an electric quadrupole located at the center on the principal plane with charges distributed at the vertices of a rhombus at the origin of coordinates. The quadrupole potential of different systems can be obtained by rotating similar potential images to different spatial orientations. Meanwhile, this paper provides the corresponding relationship between the electric quadrupole moment and the characteristic parameters of the electric quadrupole, and the equivalent electric quadrupole can be obtained from the electric quadrupole moment by solving the system of equations. The content of this paper can help students understand the physical meaning of the electric quadrupole moment better and establish a visual understanding of the electric field of the quadrupole term in the multipole moment expansion of electrostatic potential.