An inertial microfluidic chip with a spiral microchannel of asymmetric cross-section was proposed to achieve a precise control of biological microparticles/cells. The inertial focusing behavior of particles of different sizes in the microchannel was studied through simulation and experiment. A spiral channel with L shaped cross-section was designed， and the secondary flow field distribution and particle trajectory in the channel were analyzed using COMSOL simulation software. The prototype chip was fabricated by UV laser cutting and plasma cleaning bonding. The trajectories of particles of sizes 6， 10， and 15 μm at different flow rates in the channel were captured by a high speed camera and fluorescent microscope. Finally， the images of particle trajectories were stacked and analyzed and the inertial focusing and migration mechanism of the particles were investigated. The results show that two asymmetric secondary flow vortexes of different strengths are produced in the L shaped cross-section. Furthermore， the particles of 10 and 15 μm sizes focus tightly in the outer ring of the microchannel， whereas those of 6 μm size focus in it roughly. The particle focusing position can be adjusted using asymmetric secondary flow， thus providing new insights into precise particle and cell manipulation.