Two lightning flashes with bipolar currents were artifically triggered by small rockets during the winter of 1981 at Hokuriku in Japan. The lightning channels of two flashes are reconstructed by calculating both the cross-power spectrum of thunder and the cross-correlation function. The method of cross-power spectrum is more effective in the case that the acoustic waves from different channel sections arrive at the microphone array at the same time. The thunder-reconstructed channels coincide well with the actual channels under the cloud, which are reconstructed stereographically from two photographs. Charge distribution on the reconstructed channel and a sequence of discharge polarities are postulated according to the experimental results of discharge current, change of electric field at close range (70 m), surge voltage on a distribution test line at a distant of about 1 km, and framing photographs.