The focusing of particles has a variety of applications in industry and biomedicine, including wastewater purification, fermentation filtration, and pathogen detection in flow cytometry, etc. In this paper a novel inertial microfluidic device using two secondary flows to focus particles is presented. The geometry of the proposed microfluidic channel is a simple straight channel with asymmetrically patterned triangular expansion–contraction cavity arrays. Three different focusing patterns were observed under different flow conditions: (1) a single focusing streak on the cavity side; (2) double focusing streaks on both sides; (3) half of the particles were focused on the opposite side of the cavity, while the other particles were trapped by a horizontal vortex in the cavity. The focusing performance was studied comprehensively up to flow rates of 700 µl min−1. The focusing mechanism was investigated by analysing the balance of forces between the inertial lift forces and secondary flow drag in the cross section. The influence of particle size and cavity geometry on the focusing performance was also studied. The experimental results showed that more precise focusing could be obtained with large particles, some of which even showed a single-particle focusing streak in the horizontal plane. Meanwhile, the focusing patterns and their working conditions could be adjusted by the geometry of the cavity. This novel inertial microfluidic device could offer a continuous, sheathless, and high-throughput performance, which can be potentially applied to high-speed flow cytometry or the extraction of blood cells.