In order to separate microparticles of different sizes， an inertial microfluidic chip with the microchannel of triangular cross-section was proposed， and the characterization of particles focusing and separation in the microchannel was studied. Firstly， a spiral channel with right-angled triangle cross-section was designed. Then， micro-milling was used to cut the microchannel in an aluminum mold， and casting and plasma cleaning were used to fabricate the microfluidic chip. Next， sample suspensions of three different fluorescent particles （6 μm， 10 μm and 15 μm） were prepared. The trajectories of particles in the channel were captured by a high-speed camera and a fluorescent microscope， and the focusing effect of particles under the different suspension flow rates was observed. Finally， the images of particle focusing were stacked and analyzed， and the inertial focusing and separation behavior of particles were studied. The results show that with the increase of the flow rate of particle suspension， the 6 μm particles gradually focus and migrate towards the outer wall of the channel， while the focusing streams of 10 μm and 15 μm particles migrate towards the central channel. When the flow rate is set to 1.5 mL/min， the mixed suspension of 10 μm and 15 μm particles can be separated with the efficiency of 100%. This study demonstrates that the spiral microchannel of triangular cross-section produces strongly skewed secondary flow， which can be applied for the high-efficiency and precise separation of microparticles of different sizes. The research findings will provide new insights for accurate cell manipulation.