Abstract: In the context of transmission line theory, an essential difference between fitted characteristic impedance and input impedance measured by open and short circuit methods was clarified. Specifically, fitted characteristic impedance was derived by separating structural effects from input impedance measured by open and short circuit methods, and a function curve approaching characteristic impedance of cable was fitted through least squares method. Combined with mathematics and transmission line theory, it was found that a second derivative of this fitted curve exhibited nonlinear characteristics due to influence of frequency-dependent parameters such as conductor loss and dielectric loss. This was manifested as a negative slope in low-frequency region and a tendency to be horizontal in high-frequency region, presenting properties of truly reflecting characteristic impedance. Based on this, fitting processes of fitted characteristic impedance were elaborated, and fitting results were analyzed, so that a theoretical reference was provided for precise extraction of characteristic impedance and understanding of model.