The invention relates to the technical field of electric power and electric networks, in particular to a processing method and a processing system for combination power flow off-limit of transmission and distribution networks. The system comprises a boundary equipment mapping module, an off-limit equipment search module, a waiting alternative supply switch set determining module, an alternative supply scheme set determining module and a power flow off-limit processing scheme set determining module. The processing method and the processing system for the combination power flow off-limit of the transmission and distribution networks are specific to power flow off-limit of independent processing equipment of transmission and distribution networks in the existing electric power system, and can improve the power supply reliability of the electric power system; when an EMS (Energy Management System) has equipment power flow off-limit, the off-limit can be eliminated by alternative supply of a load on a distribution system side, a power failure load can be reduced, and the power supply reliability can be improved; and when a DMS (Distribution Management System) has equipment power flow off-limit, off-limit verification is performed on an alternative supply scheme of a distribution system on a transmission system side, so that a potential safety hazard of equipment off-limit of a transmission system caused by the alternative supply scheme of the distribution system can be avoided, and the power supply reliability can be improved.