A method for preparing agricultural germicid, include the steps: 4-(4-) -2-chloro acetophenone propylene glycol generate cyclics-3-chlorine-4-(2,4-dimethyl-1,3-dioxane-2-group)benz-4'-, the reaction happens with catalyst-paratoluenesulfonic acid existing in the solvent, the density mess ratio is 5-50%. When catalyst-paratoluenesulfonic acid exists, the cyclics reacts with bromine, generating bromide-3-chlorine-4-(4-methyl-2- -1,3-dioxane-2-group)benz-4'-chlorine, the bromidereacts with 1,2,4- with potassium bromide as catalysis in acutesolvent, whose density mess ratio is 15-50%, acquiring the target compound. The invention has increased the productive efficiency of by more than 20%.