Abstract: Anderson’s theory indicates that the conjugate rupture phenomenon of earthquake can be explained by Coulomb’s criterion, and there is a simple quantitative relationship between the angle of fault plane and the friction coefficient of underground rock. Based on the author’s method of calculating the friction coefficient of the medium through conjugate earthquakes, this paper systematically analyzes 19 domestic and foreign earthquakes with X-shaped or incomplete X-shaped （L-shaped） conjugate rupture characteristics. Analyze and compare with the source mechanism, regional stress axis direction and other data to determine the conjugate fault plane, and then determine the angle of the conjugate plane and calculate the internal friction coefficient of the rock. The research results further confirm that the conjugate fracture of natural earthquakes is in good agreement with the laboratory rock fracture, and the Coulomb criterion and Anderson theory are also valid at large scales. It is shown that the internal friction of the underground medium is determined by the conjugate angle. The coefficient is a useful supplement to the cognition of the characteristics of the underground medium.