This paper proposed the Baum-Liu-Tesche (BLT) method using chain scattering parameter to analyze electromagnetic coupling signals to a twisted pair over ground by HEMP. To analyze the various coupling effects between the ground and each twisted wire, Multi-conductor Transmission Line (MTL) analysis method was used. We made a TWP with many non-uniform pitch-lengths the simplified TWP-MTL model to be shown as one MTL, which reduced the calculation time and memory size when it was compared with the conventional BLT method. The results of the proposed method were compared with those of numerical analysis method and good agreements were obtained in both frequency and time domain.