The invention relates to the field of chemical synthesis, and particularly relates to a long-chain alkyl formamide which is prepared from formic acid and derivatives thereof and alkyl primary amine through a formylation rectification device. The invention further provides the preparation method of the long chain alkyl formamide. The preparation method provided by the invention comprises the step of pre-reaction treatment in the formylation rectification device, and processes of adjustment on the feeding position of the formic acid and derivatives thereof and alkyl primary amine in the device and adjustment on the device. Moreover, the invention further provides the structure of the formylation rectification device. The formylation rectification device comprises a reaction rectifying tower with a micro-reaction space. The products of the long-chain alkyl formamide, prepared by the invention, are high in purity which are all higher than 99 percent, raw materials are simple and easy to access, the cost is low, and the general applicability is wide.