The invention relates to a circuit breaker. The circuit breaker comprises a support insulator, an optical fiber current transformer and at least one arc extinguishing chamber, wherein a support insulator upper flange is arranged at the top of the support insulator, the optical fiber current transformer comprises an optical fiber current transformer coil, the optical fiber current transformer coil sleeves the support insulator upper flange, and the upper insulator upper flange is a conductor. By the circuit breaker, the current of the arc extinguishing chamber can flow through the support insulator upper flange to enable the current of the arc extinguishing chamber to pass through an inner hole of the optical fiber current transformer coil, the problem of complicated structure caused by separately arranging a cylinder in an inner cavity of the support insulator to enable the current to pass through the inner hole of the optical fiber current transformer coil is solved, and meanwhile, the problem of relatively high influence on an electric field and a magnetic field in the support insulator caused by arranging the cylinder having a conductive effect and the optical fiber current transformer coil in the support insulator is solved.