Abstract: The most common result interpretation map of transient electromagnetic tunnel geological forecast is the apparent resistivity contour map. However, compared with the apparent resistivity contour map, the apparent longitudinal conductance has better electrical characteristics of the interface identification. Study on transient electromagnetic apparent longitudinal conductance interpretation methods in the tunnel geological forecast is important. The apparent longitudinal conductance parameter is calculated by use of the equivalent conductive plane method based on the whole space theory, and the relationship among the curves of the apparent longitudinal conductance including its first derivative and quadratic differential ones is described. Then, through the establishment of the tunnel forward layered model and the example of the practical application of transient electromagnetic tunnel geological forecast, comprehensive analyses and studies on the interpretation characteristics as well as effect of the apparent longitudinal conductance are made, and the interpretation method which combines the apparent resistivity contour map the longitudinal conductance second differential imaging is discussed. Finally, some characteristics and laws of transient electromagnetic apparent longitudinal conductance in the tunnel geological forecast are summarized . It will guide the tunnel geological prediction of transient electromagnetic methods and continuously improve the accuracy of transient electromagnetic tunnel geological forecast.