For the direct measurement of very fast transient current (VFTC) due to switch operation in gas insulated switchgear (GIS), usually it will interfere the original operation or change the structure of switch.In this paper a method for calculation of transient current caused by the disconnect operation in GIS by the inverse operation of the radiation electromagnetic (EM) field is presented.A GIS is modeled by CST software which the basis is the combination of the proprietary perfect boundary approximation with the unbeatable efficiency of the finite integration technique, and all the media between the excitation source and the observation position are considered as a black box whose input is VFTC and output is EM field.A coefficient matrix is established to reflect the connection between the input and output in frequency domain, and the VFTC in frequency domain will be the result of multiplying the inverse matrix by the measurement result of EM field in the observation position.Finally the time domain form of VFTC can be obtained by the interpolation and IFFT Comparison between the result and simmlation shows the validation of this method.