A method for preparing (dimethylaminomethylene) malononitrile by using a micro reaction system. Cyanoacetamide, N,N-dimethylformamide and a catalyst are mixed to obtain a mixture, and the mixture and phosphorus oxychloride are simultaneously pumped into the micro reaction system that includes a micromixer and a microchannel reactor connected in series for continuous dehydration condensation. After adjusted to a target pH, the crude product is subjected to continuous liquid-liquid extraction with an organic solvent in a centrifugal extraction unit comprising a plurality of annular centrifugal extractors connected in series. The organic phase is collected to obtain the target product (dimethyl aminomethylene) malononitrile.