The invention discloses a power grid risk prevention and control system based on a cascading failure sequence. The power grid risk prevention and control system comprises a comprehensive importance unit, an accident severity unit and an accident risk assessment unit. The comprehensive importance unit is used for establishing the comprehensive importance factor of each element according to the structure importance factors and the state sensitivity factors of the elements in a power grid; the accident severity unit is used for carrying out processing according to the comprehensive importance factor, obtained by the comprehensive importance unit, of each element, the severity factor of the power grid running state and the severity factors of the load loss and then obtaining the comprehensive severity factor after a power grid accident. The accident risk assessment unit is used for conducting calculating according to the comprehensive severity factor to obtain the accident risk assessment factor. The power grid risk prevention and control system can completely represent the importance of different elements in the power grid and is close to the power grid practice, a risk assessment result is more reliable and more accurate, and therefore the extra manpower and material resource loss is reduced.