A simple flow microreactor system has been applied in the anionic polymerization of acrylonitrile initiated by lithium diisopropylamide. Polyacrylonitrile (PAN) with high molecular weight and narrow molecular weight distribution (MWD) has been obtained successfully in the system for the first time. A better control over the MWD of obtained PAN has achieved (1.49–1.93) compared with that in conventional lab-scale reactors (MWD = 2.00–2.46). Side reactions could be suppressed obviously even at higher temperatures, which was demonstrated by the 1H NMR spectra. Furthermore, the polymerization time could be regulated conveniently in the flow microreactor system and the living feature of the polymerization has been verified from the relationship of monomer conversion and molecular weight of PAN products obtained at different reaction time.