Microwave kinetic inductance detectors (MKIDs) are playing an increasingly role in THz astronomy. Superconducting coplanar waveguide (CPW) lines are commonly adopted in MKIDs for the resonators as well as the readout through line. It is therefore of particular interest to characterize the transmission properities of superconducting CPW lines at low temperatures. A cryogenic through-line method based on two cryogenic RF switches is proposed to characterize Nb and TiN superconducting CPW lines. On-chip calibration has been successfully carried out. Detailed results and analysis will be presented.