It is widely understood that fiber-reinforced composite structures made with 'thinner plies' can exhibit better mechanical properties. To avoid the friction to fibers in the traditional mechanical method, a new two-stage pneumatic fiber spreading system was established. To better understand the lateral-spreading mechanism of the system and optimize the relevant control parameters, CFD models of hot-air jet and turbine spreader are built with the help of multi-block technology and Euler multiphase flow method. By characterizing the distribution of air velocities in the flow fields, the influences of the boundary conditions were evaluated and optimized. The simulation results match well with the experiments, and show the feasibility and effectiveness of the multi-block technology and Euler multiphase flow method. Under the guidance of the optimization results, carbon fiber bundles have been evenly and continuously spread from 8 to 48 mm.