Abstract: A closed magnetic circuit was designed. In the circuit of speaking system, NdFeB and A3 steel were served as hard magnetic material and conducting material respectively, moreover, it has beed studied that the conducting properties of soft magnetic material of A3 low-carbon steel in different thickness. The magnetic density in the opening of the circuit was researched in different thickness of NdFeB. The experi-ment revealed that the width of opening, conducting material and hard material have much effect on the magnetic density in the opening of circuit.