The invention provides a circuit breaker support structure and a method for replacing a circuit breaker without removing old foundation, and belongs to the technical field of power equipment maintenance methods. The invention enlarges the support area of the top of the old foundation steel rod by a support flat plate, then determines the position of a support connecting piece on the support flat plate according to the position of a connecting pin of the new circuit breaker body, and accurately connects the connecting pin of the new circuit breaker body through a transition component, which caneffectively solve the problem that the old foundation steel rod and the new circuit breaker body do not match with simple structure and convenient operation; and the old connecting piece on the top of the old foundation steel rod is replaced by setting the support flat plate, the support connecting piece and the transition component, which, with no need to dismantle the old foundation steel rodsto avoid the steps of dismantling and new establishment of foundation steel rods, can shorten the replacement cycle, save resources and cost, avoid the problem of long circuit breaker replacement cycle caused by the disassembly and replacement of the old foundation, and help to reduce the workload and power outage time.