Cyclic polymers have been confirmed to possess unique properties and have potential in different applications due to the absence of free end groups. However, efficient synthesis of cyclic polymers remains a significant challenge. In this context, a versatile flash cyclization technique assisted by a microreactor (or micromixer) is presented. Unlike the conventional batch system procedure, the cyclization of linear poly(ethylene oxide) (l-PEO) can be instantly and completely realized in a micromixer. Successful cyclization can also be accomplished with an elevated flow rate of l-PEO and/or catalyst solution. Based on the flash character of the microreaction system, an upgraded setup is also constructed by introducing a circulation loop to recycle the catalyst solution. The cyclization time is therefore considerably shortened and the consumption of solvent is greatly reduced, which provides a significant improvement in cyclization efficiency.