The invention discloses a power grid safety accident risk grade online evaluation and early warning method and apparatus. According to the method, by use of online operation data of a power system, for an anticipated fault set of examination of a second defense line and a third defense line, modeling is carried out on control strategies of a second defense line control apparatus and a third defense line control apparatus, a system load loss amount, the number of lost users and the number of undervoltage plant stations are counted by use of a time domain simulation calculation result, an accident event grade is determined according to requirements of the Rules and the Regulations, and thus online evaluation and early warning of the risk grade of a power grid safety accident are realized.