The invention provides a synthetic method of 2,2-dichloro-1,3-benzodioxole-4-formaldehyde. The synthetic method comprises the following steps of: adding 3-methoxyl-2-hydroxybenzaldehyde and alkali into a protonic solvent, then dropwise adding hydrogen peroxide, and performing hydrolysis reaction; after the complete reaction, performing the after-treatment to obtain 2,3-dihydroxy benzaldehyde; adding an aqueous solution of the 2,3-dihydroxy benzaldehyde and alkali into the protonic solvent, and then adding dichlorodifluoromethane to perform the reaction; and after the complete reaction, performing the after-treatment to obtain the 2,2-dichloro-1,3-benzodioxole-4-formaldehyde. According to the synthetic method, the target product can be obtained from an initial raw material 3-methoxyl-2-hydroxybenzaldehyde by virtue of two-step chemical reaction. The reaction method is moderate in reaction conditions, simple in steps, low in cost, environment-friendly, high in yield, capable of facilitating the industrialized mass production, and capable of setting a foundation for the subsequent successful synthesis of fludioxonil, so that the synthetic method has good application prospect and market potential.