The invention provides an extra-high-voltage direct-current-line expected failure active standby capacity assessment method and an apparatus thereof. The method comprises the following steps of determining a maximum active standby capacity which can be provided by a standby set; calculating sensitivity of dangerous equipment in a dangerous equipment list; and forming a safety constraint and determining a maximum active standby capacity of an extra-high-voltage direct-current receiving-end power grid under the safety constraint. In the technical scheme, sensitivity analysis is performed on the dangerous equipment, simultaneously influences of direct current loss power and same increase power of the set on the equipment are considered, the sensitivity of the dangerous equipment is calculated and a sensitivity result is not influenced by balancing machine selection. And simultaneously the safety constraint is considered so that a problem that a power grid safety constraint is not considered during existing active standby capacity calculating is solved. In the invention, based on solving of the maximum active standby capacity of optimization calculating, under the condition that an active trend of the dangerous equipment is guaranteed to be in a safe range, the maximum active standby capacity which can be provided by the extra-high-voltage direct-current receiving-end power grid under different time scales can be acquired.