For the big error of characterizing the field coupling into cylindrical cavity of lightning indirect effects by using transmission method under the condition of irregular cylindrical cavity, the Richard time domain model of transfer impedance is improved. A new time domain model of transfer impedance basing on curvature radius of cavity surface is presented based on the analysis of the principle of regular cylindrical cavity’s transmission field. This model considering the influence of the radius’s change of cylindrical cavity can characterize the time domain transfer impedance of both the regular and irregular cylindrical cavities. The error between the transmission field and internal field of irregular cylindrical cavity decreases from 10.83% to 4.9% in the confirmatory experiment, and the suitability problem of transmission method used in the lightning field coupling into irregular cylindrical cavity is solved. This work provides a more widely applicable method for coupling of lightning impulse into an irregular cylindrical cavity.