The invention belongs to the technical field of extra-high voltage DC engineering AC fileds, and in particular relates to a last circuit breaker judgment method and device of a bus tie switch-involved AC field. A last circuit breaker is searched by building an equivalent model of the bus tie switch-involved AC field and a last circuit breaker graph theory solution model; when the numbers and the access positions of bus tie switches, isolation switches, circuits and valve banks of the AC filed change, an original complicated control protection logic program does not need to be modified, the system device does not need to be powered off to update the program, simple configuration parameters only need to be changed on line, and the last circuit breaker between all input circuits and each converter valve can be rapidly, intelligently and accurately calculated through acquiring switch-on and switch-off states of the current circuit breaker of the AC field in real time by the system.