Breaking through technical barriers and cost reduction are critical issues for the development of microfluidic devices, and both rely greatly on the innovation of fabrication techniques and use of new materials. The application of 3D printing definitely accelerated the prototyping of microfluidic chips by its versatility and functionality. However, the resolution of existing 3D printing techniques is still far below that of lithography, which makes it difficult to work on the scale of single cells and near impossible for single molecule work. In this paper, we present a facile way to increase the resolution of 3D printed microstructures to minimally 4 μm by casting-shrinkage cycles of a polyurethane (PU) polymer. A water-PU liquid mixture poured on a 3D printed template quickly solidifies replicating the structures, which then isometrically shrink to half its size after solvent evaporation, downscaling the replicated structures. By repeating the casting-shrinkage cycles, we could downscale the (sub)millimeter structures of 3D printed structures on demand, until the working limit posed by the polymer properties, which we demonstrate by fabricating a micromixer. Moreover, we can even fabricate microfluidic chips from millimeter-scale manually assembled templates, fully independent of any micromachining facilities, significantly reducing the technical barriers and costs, thus opening up the microfluidics field to low-resource areas.