The invention discloses a method for online estimating and pre-warning the grid safety accident risk levels, and belongs to the technical field of power system automation. The method comprises the following steps: on the basis of a grid current operation manner provided by an EMS and a control strategy of a second defensive line and a third defensive line, calculating and simulating the action of a control device of the second and third defensive lines through time-domain simulation in allusion to a preconceived fault set of assessments of the second and third defensive lines, so as to obtain a total fault load reduction quantity, a loss user quantity and a voltage loss station quantity; dividing the grid safety accident risk levels of the preconceived faults according to the requirements of ''Regulations on the Emergency Response to and Investigation and Handling of Electric Power Safety Accidents '' and ''Rules for the Investigation of Electric Power Accidents of China Southern Power Grid Company Limited''; and determining the important levels of the faulted equipment according to the accident risk levels. According to the method, the online estimation and pre-warning of the grid safety accident risk levels are realized, a technological foundation is laid for the real-time pre-warning and pre-control of the grid safety accidents.