The invention provides a differential protection method and system for a multi-end T-connection power transmission line and belongs to the differential protection technical field. Two slaves are selected so as to be connected together to form a slave unit; a target slave and an auxiliary slave are determined from the slave unit, a channel A, a channel B, and a channel C are sequentially connectedwith one another so as to form a first channel, and the channel C, the channel B and the channel A are sequentially connected with one another so as to form a second channel; the target slave sends two frames of messages, the first frame of message and the second frame of message are respectively transmitted to the target slave through the first channel and the second channel, and delay differencebetween the receiving-transmitting delay of the first channel and the receiving-transmitting delay of the second channel is obtained; and if the delay difference is larger than a preset difference value, a master and the target slave give out an alarm, the target slave quits from differential protection. According to the differential protection method and system for the multi-end T-connection power transmission line of the invention, one channel is added between the target slave and the auxiliary slave, the time different value is obtained on the basis of the added channel, and therefore, thechannel receiving-transmitting delay of the master and the target slave can be monitored.