The invention discloses a whitewashing agent applied to the protection of tree trunks and application. The whitewashing agent comprises the following components in percentage by weight: 40 to 45 percent of calcined lime, 2 to 3 percent of lime sulfur mixture stock solution, 3 to 6 percent of lime sulfur mixture residues, 2 percent of salt, 0.5 percent of grease and the balance of water, wherein the lime sulfur mixture residues are a mixture which mainly contains silicon oxide. A preparation method for the whitewashing agent comprises the following steps of: adding the water and lime hydrate into a reaction kettle, and heating and stirring continuously for later use; putting the calcined lime into a hot-water boiler, and dissolving to prepare lime milk solution, putting sulfur powder which is blended to form paste in advance, stirring and mixing while adding the pasty sulfur powder for 40 to 60 minutes until liquid medicine has a rufous color, and the liquid surface has a layer of membrane, cooling the liquid medicine and collecting supernatant; and mixing the two liquid mixtures, heating, stirring and cooling to obtain the whitewashing agent. By applying the whitewashing agent, fruit tree barks can be prevented from being cracked, the resinosis can be avoided, the day-night temperature difference of branches can be reduced, and pathological changes caused by cold damage and insect attack can be prevented effectively, so the whitewashing agent can be used in various fields.