The invention relates to a B-dot probe blocking electrons of a magnetically insulated sheath and a current space distribution measuring system. The B-dot probe comprises a magnetic induction coil, a cable connector and a supporting and fixing device, wherein the magnetic induction coil is wired on a circuit board clockwise or anticlockwise in the same direction to form a PCB type magnetic induction coil; and the supporting and fixing device comprises a metal clasp and a metal support ring, and a stepped hole is formed at the position, just facing the top of the PCB type magnetic induction coil, at the top of the metal clasp. An insulated slice of high-molecular material is arranged in the stepped hole and fixedly connected with the metal clasp, when the B-dot probe is bombarded by electrons of the sheath of a magnetically insulated transmission line, the electrons are blocked by the insulated slice, and energy of the electrons is accumulated in the insulated slice.