This study presents a harmonic transfer function (HTF) based single-input single-output (SISO) impedance modeling method. The method converts an HTF from phase domain to sequence domain and then transforms it into an SISO impedance while preserving the frequency coupling information of different sequences and different harmonics. Applications of this method to a line-commutated converter based high-voltage direct current (LCC-HVDC) system are presented. The results demonstrate the accuracy of the derived SISO impedance, and a truncation-order selection is suggested. The case study shows that the proposed method facilitates simpler impedance measurements and associated stability analysis.