The present invention relates to a multi-terminal line differential protection method and system. The differential protection method includes the following steps that: the protection sub-machines of a plurality of terminal lines acquire the electric quantity data of the corresponding terminal lines and send the electrical quantity data to a protection main machine; the protection main machine receives the electrical quantity data sent by the protection sub-machines, performs differential protection calculation, and sends tripping commands to the protection sub-machines if an in-area fault occurs; and the protection sub-machines control circuit breakers at the corresponding terminal lines to trip according to the tripping commands so as to remove the fault. According to the multi-terminal line differential protection method and system of the invention, one protection sub-machine is arranged at the terminal of each terminal line, and the protection main machine is arranged at a certain terminal line, the protection sub-machines acquire the electrical quantity data at the corresponding terminals and sends the electrical quantity data to the protection main machine; the protection main machine acquires the data sent by the protection sub-machines, performs differential current calculation to judge whether a fault is an out-area fault or an in-area fault, and sends the tripping commands to the protection sub-machines if the in-area fault occurs; and the protection sub-machines trip the circuit breakers at the corresponding terminal lines through the tripping loops of the protection sub-machines according to the tripping commands, and therefore, the isolation of the fault is completed.