The invention discloses a compound bactericide and a preparation method. The compound bactericide is prepared from a dimethachlon active compound, salicylhydroxamic acid, sodium dodecyl benzene sulfonate, sodium lignin sulfonate and diatomite or bentonite in a certain ratio. The preparation method comprises steps as follows: 1) the dosage is calculated according to the prepared dosage form requirement, the active compound, salicylhydroxamic acid, sodium dodecyl benzene sulfonate, sodium lignin sulfonate and diatomite are weighed, placed in a container, evenly mixed and crushed in a crusher; 2) a crushed sample is evenly mixed and crushed by an airflow crusher to a certain fineness (95% of the sample passes through a 320-mesh sieve), and dimethachlon wettable powder is obtained after even mixing. The compound bactericide adopts a reasonable formula, is convenient to use, has an obvious synergistic effect on dimethachlon in preventing sclerotinia rot of colza, can remarkably improve the dimethachlon control effect, is safe and free of pesticide damage to crops, can reduce the dosage of dimethachlon, reduces the control cost and is expected to delay production of drug resistance of dimethachlon.