The invention discloses a fixing agent. The fixing agent comprises 0.01-0.02 M of sodium dihydrogen phosphate dihydrate, 0.01-0.02 M of sodium dihydrogen phosphate, 0.05-0.15 M of sodium chloride, 4-6% of paraformaldehyde, and the balance of water. Further, the pH value of the fixing agent is 7.0-7.4. A preparation method of the fixing agent comprises the following steps: 1, accurately weighing sodium dihydrogen phosphate dihydrate, disodium hydrogen phosphate and sodium chloride into a beaker, adding a proper amount of ultrapure water, and uniformly stirring and mixing for sufficiently dissolving; 2, accurately weighing paraformaldehyde into a beaker, adding paraformaldehyde into the solution prepared in step 1, heating and stirring with a magnetic stirrer, controlling the temperature tobe 60 DEG C or below until paraformaldehyde is completely dissolved, and adjusting the pH value to 7.0-7.4 with a 10N NaOH solution; and 3, after the pH is adjusted, adding ultrapure water into the beaker by using a measuring cylinder, fixing the volume to a preparation volume, uniformly stirring and mixing, and storing in a dark place at room temperature. The advantages of good detection specificity and sensitivity, low detection cost, high stability, simple operation, high repeatability, and guaranteeing of the reliability of the result are achieved.