The invention provides a method to detect the trace Cl- in the limestone serum by the mercuric nitrate titration. The Cl- reacts with the Hg+ of the mercuric nitrate to form the mercury chloride. When the titration finishes, the redundant Hg+ reacts with the indicator diphenylcarbazone to form the purple complex compound, so the solution color changes from the Kelly to the purple which is the end point. Last to compute the Cl- content according to the titer of the mercuric nitrate. The invention has the simple operation and low cost, it can detect the Cl- of some ppm and get rid of interfere of the sulfurous acid radical, the titration terminal has the evidence color change and easy to distinguish.