Microprocessor based protection has been widely applied in power system recently. However, part of them can not satisfy the local needs of the power system under some operation modes in practical application. Mode of connection of short double-circuit transmission lines increase gradually along with the development of urban power grids and the probability of protection false action add as well in combined failure. Therefore, in order to avoid protection false action and accident enlargement when electric network special failures happen,the setting value of X’ and R’ must be comprehensively considered based on the zone of protection in the opposite direction ,the transform errors of current transformer and potential transformer, the measurement error under exactitude operating voltage etc and rationally chosen according to the mode of connection. This paper analyzes the action about CSL-162B microprocessor based protection in combined failure of double-circuit transmission line, to indicate the existing problems of this protection, and to submit the improvement measures to prevent the occurrence of same kind faults.