A method for detecting ammonia nitrogen of a low mercury salt belongs to the technical field of chemical engineering. The method comprises the following steps: preparing a mercury bichloride-potassium iodide-potassium hydroxide solution used as a Nessler's reagent, preparing a potassium sodium tartrate solution, preparing an ammonia nitrogen solution in colorimetric tubes to obtain an ammonia nitrogen standard series, and drawing a calibration curve. The method for determining ammonia nitrogen in wastewater, which uses HgCl2 to prepare the Nessler's reagent, has no precipitate generation, realizes the high accuracy and the high precision of a determination result, and can be used for control analysis. Comparison researches confirm that the detection of the ammonia nitrogen through the Nessler's reagent prepared from KI, KOH and HgCl2 and the optimized potassium sodium tartrate solution guarantees the low content mercury and is fast and simple, and the calibration curve made through the method has a high sensitivity.