[en] In this paper the old longitudinal coupling impedance measurement system of National Synchrotron Radiation Laboratory is reviewed briefly. Considering the deficiency of the old system and advance of measurement apparatus, the improvements of measurement principle and hardwares were made. The measurement principle, system hardware configuration of new improved measurement system based on coaxial-wire method is presented in details. The measured impedance is agreement well with the calculated impedance by ABCI code, and the performance of the new system was demonstrated