Abstract: To fix the problem of low sensitivity of the passing-through eddy detection of medium/large-sized steel tubes,major elements responsible for detection sensitivity are analyzed.Following the principle of electromagnetic equal effects,the electromagnetic equivalent diameters of the excitation coils of the passing-through type probes under different exciting modes are investigated.The result shows that under the constant current exciting mode,the exciting magnetic field is the strongest and has a focusing effect,which improves the eddy detection sensitivity considerably.