Water-in-water (W/W) droplets are biocompatible vessels for bioanalysis and biomolecules delivery. Due to the ultralow interfacial tension, the stable generation of W/W droplets still faces some challenges. In this paper, we present a robust and high-throughput microfluidic platform to fabricate W/W droplets without requiring external perturbation. Using the assembled evaporation pump, W/W droplets are generated uniformly and stably for nearly 1 h. The molecular weights, concentrations, and flow rates were changed to regulate the droplet size in the range of 44-93 mu m. The production of droplets is scaled up by parallelizing eight droplet-formation units on a 3-D microdevice, and the variable coefficient of droplet size in all units reaches 3.2%. Using these W/W droplets as microreactors, monodispersed hydrogel particles are synthesized by either UV light or calcium ions, and recovered conveniently without cumbersome post-processing. The established method is simple, robust and suitable for various aqueous two-phase systems, displaying its potential in biocompatible carrier synthesis.