Abstract: The calculation method of apparent longitudinal conductivity in layered media is discussed in this paper based on transient electromagnetic theory of the floating thin sheet. Genetic algorithm is used instead of damped least square method to get the key parameter. The smoke loop inversion is also calculated, so that the curves of apparent longitudinal conductivity can be compared with the resistivity-depth curves in the typical geoelectric model of 3 layers H, K type and 4 layers HK type. The results show that although the smoke loop inversion is easy to realize and needn't to set initial model in qualitative inversion area, the interpretation of apparent longitudinal conductivity can distinguish electrical layers more precisely, and it can also indicate the change trend of the electrical property directly.