Abstract: The report represents a method to determinate trace levels of chloride in sodium coolant in a fast neutron reactor. Vacuum distillation is used for separating chloride from sodium metrix. After separation, metrix interference from sodium salts is eliminated and a large sample amount can be taken for analysing chloride. In contrast with other similar mercuricthiocyanate colorimetric method in China, this method minimizes a glaring blank of colorimetric solution and determining influence of temperature. Therefore, it increases the precision in analysing chloride. The results of the research demonstrate that when a level of chloride in sodium is less than 1 μg/g, the precision is 37 %( n =10). The recoveries of chloride in high pure sodium demonstrate that when 15～30 μg/g of chloride is added in sodium, the recovery is 97.8 %～98.6 % and when 4 μg/g of chloride is added, then the average recovery is 99.2 %. And the relative standard deviation is 16.2 %( n =7). It is demonstrated that a monitoring requirement of chloride can be satisfied with this method in coolant of sodium in a fast neutron reactor.