Strong electromagnetic pulse can induce nanosecond rising edge pulse conduction disturbance on the cable in the form of field-transmission line coupling, which poses a great threat to the equipment at the end of the cable. For a certain type of relay protection device, the immunity performance is tested first, and then the high-altitude electromagnetic pulse irradiation test under the field-line coupling path is carried out to obtain the coupling characteristics of the device port. The device malfunctions when the common mode current coupled to the signal port reaches 32.45 A and above. At the same time, the pulse current injection test is carried out. When the pulse current injected into the signal port reaches 36.92 A or more, the device malfunctions, further confirming the critical interference threshold of the device port. Through the establishment of the field-line coupling model of the secondary cable in the substation and the signal cable in the protective panel cabinet, the coupling quantity of high-altitude electromagnetic pulse in different scenarios is calculated, and the key points of field-line coupling protection are proposed. The research results can provide reference for the evaluation of anti-interference ability and protection technology of relay protection devices in strong electromagnetic pulse environments.