Inertial focusing of microscale particles has increasingly attracted attentions in various microfluidic applications, such as manipulations of particles/cells, flow cytometry, and highsensitive detection, owing to its passive, robust, and high-throughput nature. While most fundamental studies in literatures have focused on the lateral migration and transverse equilibrium position of single particles in micro-channels with various cross sections, very few of them have dug into the forming and ordering mechanisms of an inertially focused particle train. In this study, the forming inertially focused particle trains are investigated by coupling the lattice Boltzmann method with the immersed boundary method (IB-LBM). The inertial migrations of 6 particles (40 μm in diameter) are simulated in a square microchannel (100 μm in width). The results indicated that the equilibrium axis distance between two neighboring particles is strongly affected by complicated particle-to-particle and particle-to-liquid interactions. Two critical length fractions Lf,c1 and Lf,c2 are observed, which have insignificant effect on balanced distance between two neighboring particles in a particle train.