Abstract: The lanthanum ions in the LiCl-KCl eutectic with 0.98 %, 2.0 % and 3.3 % LaCl₃ were tested by CP electrochemical method in the 773 K argon atmosphere. The results show that the reduction potential of La (Ⅲ) on the tungsten electrode relative to the silver/silver chloride reference electrode was about -2.0~-2.2 V. By calculating the relationship between the value of the cathode peak current, transition time and the immersion depth in the CP results, the diffusion coefficient was measured to be 1.29×10^-5~5.42×10^-5 cm²/s at 773 K. The relative errors compared Chronoamperometry (CP) and Inductive Coupled Plasma Emission Spectrometer (ICP) of La ion concentration determination is 1.25 %, 1.11 % and 1.72 % respectively. In LiCl-KCl eutectic, the peak root current and transition time square root product of La electrodeposition show a good linear relationship with the concentration. It is feasible to apply the CP electrochemistry techniques directly to determination the ion concentration.