Abstract: Molten-salt electrorefining is a key step in the pyroprocessing of spent nuclear fuel. During electrorefining, actinides and fission products gradually accumulate in the electrolyte, and the composition of the molten salt changes continuously. Such changes can influence current efficiency, deposition behavior, and product purity. Reliable on-line analysis of representative ionic species is therefore of practical importance for process monitoring and control. In this work, samarium was selected as a representative rare-earth fission product, and its quantitative determination in a SmCl₃-LiCl-KCl eutectic melt at 773 K was investigated by square wave voltammetry(SWV). The electrochemical behavior of the Sm³⁺/Sm²⁺ couple was first examined in order to identify the characteristic reaction region and to evaluate whether SWV could be used under high-temperature molten-salt conditions. The results show that the Sm³⁺/Sm²⁺ redox process can be treated as a quasi-reversible electrode reaction within the potential range adopted in this work. On this basis, SWV measurements were carried out for melts containing 1%-8%(mass fraction, the same below) SmCl₃. Distinct and stable voltammetric peaks were obtained throughout the whole concentration range, indicating that SWV is capable of providing a useful analytical signal for Sm(Ⅲ) in the chloride melt. In the low-concentration range, especially below 4% SmCl₃, the peak current density shows a good linear relationship with concentration. When the SmCl₃ concentration exceeds 4%, however, the response gradually deviates from linearity. This result indicates that the mass-transfer behavior in the concentrated melt is no longer as simple as that in the dilute region. To further clarify the origin of this deviation, the contribution of non-faradaic current was examined. Background-current measurements performed in a potential region without faradaic reaction show a clear dependence on SWV frequency, which means that the charging current cannot be neglected in quantitative analysis. After background-current correction, the calibration curve in the 1%-4% range is markedly improved, and the fitted relationship between peak current density and concentration give an r² value of 0.9993. The corrected calibration curve is then used for in situ determination of Sm(Ⅲ) concentration. The values obtained by SWV agree well with those measured by inductively coupled plasma atomic emission spectrometry(ICP-AES), and the deviation is less than 5%. The results demonstrate that SWV, when combined with appropriate background-current correction, can be used for reliable quantitative analysis of Sm(Ⅲ) in LiCl-KCl molten salt. This method may provide a practical basis for the on-line electrochemical monitoring of rare earth fission products during molten-salt electrorefining and may also be useful for process analysis in pyroprocessing systems.