Particle focusing induced by viscoelasticity of fluids has attracted increasing interest in recent years. However, the regulation mechanisms of critical parameters affecting the particle focusing behaviors are still unclear. This paper systematically characterized the dynamics of particle migration in non-Newtonian fluid flows, and analyzed the effects of flow rate and channel length on particle focusing behaviors. Först, the lateral migration behaviors of particles suspended in Newtonian fluids (e.g., pure water and 22 wt% glycerol aqueous solution) are compared with those in non-Newtonian fluids (8 wt% polyvinylpyrrolidone aqueous solution). It is found that the particles suspended in non-Newtonian fluids would migrate towards the channel centerline and form a single-line particle array under the action of elastic force while the particles suspended in Newtonian fluids would migrate to form a famous Segré-Silberberg particle annular ring due to the effects of inertial lift forces. Second, the effects of particle size and driving flow rate on particle viscoelastic focusing are quantitatively analyzed. Results show that with increasing flow rate the focusing degree increases and finally stabilize at a certain value, and the large particles have better focusing quality than the small ones. Finally, the dynamic focusing process of particles along the channel length is investigated. A mathematical model of safe channel length for achieving particle focusing is derived and validated by experiments. It is found that the safe channel length for large particles is significantly shorter than that for small ones. The obtained results would improve the understanding of particle focusing processes and mechanisms, and help realize the flexible control of particle migration behaviors in non-Newtonian fluids.