An operating mechanism of a small-size circuit breaker comprises a handle, a transmission rod, a support piece and a movable contact, wherein the support piece is arranged in a shell and can do rotation movement, the support piece and the movable contact are pivoted and arranged on the same common shaft, one end of a spring is fixed on a base, the other end of the spring is connected with the movable contact, the handle drives the support piece to rotate by the transmission rod, the support piece pushes the movable contact to rotate, and the movable contact can rotate around the same common shaft with the support piece and also can slide relative to the support piece. By the operating mechanism, the number of parts can be reduced, the manufacturing process can be reduced, e.g. an original over-range mechanism and an over-range spring are omitted, and the volume space of the circuit breaker can be further reduced.