The invention provides a synthesis method of m-nitrobenzaldehyde. The method comprises the following steps: firstly, reacting, concretely carrying out reaction on copper nitrate, water, silver nitrate and benzaldehyde in a molar ratio of (1.03-1.1):(20-40):(0.001-0.0012):1.0, so that reaction mixture is obtained; secondly, preparing an m-nitrobenzaldehyde crude product, concretely, filtering while the reaction mixture is hot, so as to obtain mother liquor, and then cooling the mother liquor, filtering, and taking solid, so that the m-nitrobenzaldehyde crude product is obtained; and thirdly, preparing high-purity m-nitrobenzaldehyde, concretely, adding a water soluble solvent into the m-nitrobenzaldehyde crude product, filtering, and drying, so that high-purity m-nitrobenzaldehyde is obtained. By adopting the method provided by the invention, molar yield of the obtained m-nitrobenzaldehyde is more than 90%, chromatographic purity is more than 99.8%, and content of o-nitrobenzaldehyde is less than 0.1%; and fewer three wastes are produced in a production process, so that the method provided by the invention is environmentally friendly.