The invention provides an online evaluation method for the safety control amount of an AC line fault based on the speed-down DC sensitivity. The method comprises the following steps that whether offline data is calculated for the first time or the operation state of an AC line is changed is determined, if YES, the offline data is updated, and otherwise, the offline data is not changed; the overload quantity of a key AC line is determined; and the speed-down DC safety control amount is calculated, and a limited AC line is determined. According to the method, a safety control measure for an N-2 fault of primary and secondary sectional lines of a DC converter station can be calculated online automatically rapidly, a calculation result is accurate, the intensity of manual calculation can be reduced greatly, the working efficiency of power-grid staff is improved, and the method is suitable for a reception-end AC system which has a problem in trend evacuation due to limitation of heat stability of near-area lines after large-capacity DC is fed in.