Abstract: A differential mode current injection method substituting for radiation is proposed for interconnected systems，with the antenna and the cable as the main coupling paths.The effective upper frequency limit of current injection is raised from 400 MHz to gigahertz，and the accuracy of the method is also confirmed when the equipment under test(EUT)works in the nonlinear region.The coupling device which is composed of directional couplers can inject differential mode currents with specific direction and monitor the positive traveling voltage at the EUT input port.The equivalent relationship between the radiated field strength and the injected voltage is analyzed.It is verified that this relationship is linear when the response of the auxiliary equipment and the EUT are linear and nonlinear，respectively.Therefore，linear extrapolation is employed to obtain the injected voltage which is equivalent to highlevel field strength.The voltage at the monitoring port of the device is treated as the equivalent parameter. Thus， this method is also effective when the EUT is a black box.With a typical antenna and radio frequency frontend components under test，the feasibility and accuracy of the method are verified.