According to above analyses and considering the applicability under special conditions, a mathematical expression (Equation 4) was put forward to represent the improper degree of facility damage risk/ human injury risk. From the equation, it can be known that if facilities’ ranking number of importance matches well with the ranking number of the damage risk (can be replaced by the individual risk), the improper degree of facility damage risk/ human injury risk will be low, and the lower the damage risk, the lower the improper degree of facility damage risk/ human injury risk.