The method of determining of sodium chloride in the rock salt through potassium dichromate backtitration is studied. A fixed ration of K2Cr2O7-H2SO4 reacts with rock salt under the heating condition, and chromyl chloride is produced. Upon evaporation of the produced chromyl chloride, the standard ammonium ferrous sulfate liquid is used to titrate the rest of K2Cr2O7. The relative standard deviation of determination results is 1.35%, and the recovery rate of add standard is between 97.5%-100.2%. The results show that the improved method is accurate, quick, with little disturbance, and end point to be effective. It is an accurate and fast way of determining chlorides content, which not only can be used to determine the content of chlorine in various dissolvable chlorides, but also can be used in experiments intended more quickly and accurately.