The precise control of hydrogel shaping at high resolution is an intrinsic engineering challenge due to the low solid content of the material. Most of the approaches are based on lithographic mold, which becomes tricky to demold when the precision rises to micron and sub-micron range. In this work, a novel automated “electrodeposition + pulling” approach for rationally shaping chitosan hydrogel in 3D is reported. Taking advantage of the transient meniscus generated while pulling a sharpened metal wire out from a viscous solution, the process confines the electrodeposition of hydrogel simultaneously in it. Considering that the electrodeposition of hydrogel is driven by electrochemically generated OH- ions, the reaction engineering rationally and synergistically combines the mass transport of OH- ions, the gelification and the fluid mechanics in a dynamic process. With automation and mastering the process, the shape of hydrogel can be tuned, and it can be sharpened to below 1 micron at the tip with a desired length. Thus, it is demonstrated for electrochemical writing on 3D printed complex shaped surfaces. The sharpened hydrogel is in soft contact with the sample, allowing electrochemical deposition or etching reactions to be spatially localized in the contact area. By precisely pressing or stretching the hydrogel, the resolution can be tuned from a few to tens of microns just like traditional ink brushes, which may significantly improve the efficiency of writing. The work shows the power of electrochemical reaction engineering towards spatial control in 3D at micro and nano scale.