In a UHV transmission system, there is an abnormal working condition in which the single or multiple interval line differential main protections are lost. If the fault protection in the line area after the main protection is lost, there may be a long action delay, mismatch or mis-operation risk caused by mismatch. The adaptability of back-up protection is analyzed in view of the loss of line differential main protection caused by single or multiple line protection device blocking, communication equipment abnormalities and communication channel failure. An identification logic, for judging the loss of main protection caused by different operating conditions, based on the collection of switch information in the substation domain is proposed. An emergency optimization plan for active and passive acceleration self-adjustment of backup protection based on the shared information of the station domain is formulated to shorten the operation time of the backup protection and adaptively adjust the range of the backup protection. This is verified by simulation. The simulation shows that the optimized emergency backup protection shows that distance protection and zero sequence protection can quickly and, in an orderly fashion, remove the faults after line differential main protection is lost. This work is supported by the Science and Technology Project of State Grid Central China Branch (No. SGH80000DKJS1900219) and the Science and Technology Project of the Headquarters of State Grid Corporation of China (No. 5100-201919020A-0-0-00).