An innovative coaxial capillary device with a size-comparable outlet channel was designed to meet the challenges for controllable production of highly monodisperse millimeter-sized double-emulsion droplets by one-step dripping. The technique of two angled junctions, coaxial capillaries, and compound channel geometry was introduced to convert the millifluidics to microfluidic hydrodynamic features. The relevant nondimensional numbers for a typically regular dripping in this kind of millifluidic flows were similar to those in common microfluidic devices. Effects of the rival forces due to the fluid factors on droplet formation were clarified to explain the dynamic behaviors of double-emulsion formation and to establish the mechanism of droplet breakup. The results helped to understand the coaxial flow pattern for obtaining a more precise droplet size control and to develop high-throughput setups for chemistry, physics, and biology.