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The evaluation of the electromagnetic field radiated by a lightning stroke channel is a necessary step for the analysis of many electromagnetic compatibility problems. An efficient algorithm for calculating the electromagnetic field of a lightning stroke channel is presented in this paper. The electromagnetic field of a lightning stroke channel, expressed by the generalized Sommerfeld integral, is decomposed into two parts. One is a definite integral, which can be evaluated using the proposed moment technique to avoid the troublesome oscillatory integral; the other is an improper integral with an infinite upper limit, which is calculated with the newly developed asymptotic expression to reach convergence quickly. The proposed approach turns out to be very efficient. The numerical examples validate the proposed approach in both frequency and time domain.