The invention discloses a power system online operation safety risk assessment method based on extreme value theory. The method is mainly characterized by, through limited-quantity simulation, carrying out operation safety tail risk modeling by utilizing the extreme value theory in statistics, so that possible system operation risk situations having large influence can be estimated effectively; and the method can accurately describe possible risk factors in the operation process in the model with various possible uncertain factors of new energy (wind power and photovoltaic and the like) forecast errors and load fluctuation and the like, which may appear in the system operation process, being taken into consideration. Besides, the method only depends on limited samples, so that the method has a rapid calculation effect; and the method is a practical on-line assessment method, can be applied to online safety risk assessment of a real system and provides valuable reference and guidance for safe operation of the system.