Inertial microfluidics systems carry a great importance for particle focusing and separation due to their simple configurations, robust structures, and precise high-throughput cell manipulation ability. In this paper, a novel microchannel geometry, called Sunflower, which is developed by superimposing two microchannel geometries, is presented. An empirical formula for normalized focus quality, which enables the quantitative comparison of particle focusing abilities of different microchannel geometries via fluorescent images, is introduced. In particle focusing experiments, focus quality enhancements ranging from 14% at 0.9 ml min−1 to 487% at 0.3 ml min−1 is observed. Besides, 99.57% of the 9.9 µm polystyrene particles are collected at a flow rate of 2.5 ml min−1 from a 60 µm wide outlet section, whose location within the microchannel width is specifically tailored. A clear, stable and narrow particle focus line is observed not only at an optimum flow rate, but also in a great range of flow rates between 0.3 ml min−1 and 2.5 ml min−1, offering great levels of flexibility and robustness against disturbances. Moreover, the novel microchannel geometry is tested with HL-60 cancer cell lines, and similar results are obtained in a great flow rate range without an observed effect on the cell’s viability.