As a kind of important semiconductor material, crystalline silicon has vast applications in many industries, such as integrated circuits and solar cells. With anisotropic etching method, including alkali etching and copper assisted catalytic etching, pyramid or inverted pyramid structure on the surface of silicon can be formed due to different crystal face indices of the silicon wafer, which is especially for multi-crystalline silicon wafers, because there are many different crystal faces on the surface. The proportion of different crystal faces has a high reference value for controlling the quality of multi-crystalline silicon. In this paper, the mathematical model of the inverted pyramid structure is established by making use of the relationship between the silicon crystal indices (abc) and {111} crystal plane. The inverted pyramid structures with different crystal face index (abc) values are divided into three possible cases for discussion, which are 0≤a≤bc, 0≤ab=c, a=b=c. The inverted pyramid structure on which the crystal face index (abc) satisfies 0≤a≤bc is of a pentahedron composed of five points and has a quadrangular cross section. The inverted pyramid structure in which the crystal face index (abc) satisfies 0≤ab=c is of a heptahedron composed of eight points and has a hexagonal cross section. The inverted pyramid structure whose crystal plane index (abc) satisfies a=b=c=1 is also of a heptahedron and has a hexagonal cross section but is composed of nine points. In general, the cross section of the (111) crystal face inverted pyramid is similar to an equilateral triangle because three of the edges are easier to etch away. The scanning electron microscopy image results show that the crystal indices are (100), (110) and (111), thereby demonstrating the correctness of the theoretical calculations. The index of crystal face has a one-to-one correspondence relationship with the inverted pyramid structure. Therefore, according to the inverted pyramid structure after anisotropic etching, we can measure the index of Si crystal face.