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• Stable and homogeneous of droplets for the aqueous two-phase system is achieved. • Neck evolution of water-in-water droplets in flow-focusing devices is self-similar. • Four flow patterns were observed and analyzed. • Formation process of droplet includes slow and fast squeezing, and pinch-off stages. • Evolution regulations of droplet neck interface in three stages were described. In ATPS, the ultra-low interfacial tension makes the stable generation of droplets very difficult. In this paper, the stable and homogeneous preparation of ATPS droplets is achieved over a wide range of flow rates, and the self-similarity analysis and formation dynamics of the droplets are studied. Formation of ATPS droplets show good self-similarity. The generation process of ATPS droplets can be divided into slow squeezing, fast squeezing and rapid pinch-off stages. In the slow squeezing stage, the dimensionless minimum neck width w m / w c is linear with the time, and the absolute value of the slope increases with the capillary number of continuous phase Ca c. In the rapid squeezing and rapid pinch-off stages, the w m / w c satisfies the power-law relationship with time, and the absolute value of the power-law indexes decrease with increasing Ca c , and the pre-exponential factor of pinch-off stage increases with increasing Ca c. [ABSTRACT FROM AUTHOR]