Based on the powder-binder two-fluid model, the filling process was simulated by CFX commercial software, especially for the ultra-fine WC/10Co round bar with large aspect ratio by powder injection molding (PIM). The results indicated that the numerical simulation results are almost consistent with the experimental filling process, which proves that the hypothesis and parameter settings are reasonable and the two-fluid model is feasible. Also there is no great difference in temperature distribution between powder and binder. There is no solidification for melting feedstock, which due to that the lowest temperature valued 330 K of melting feedstock is higher than the vitrification temperature of binder. In addition, there is great difference in the viscosity between powder and binder, valued 50.0 ~379.4 Pa·s, 2.9~9.2 Pa·s, respectively, which is one cause for segregation. Finally, it is also another main cause for segregation that the relative velocity difference between powder and binder increases up remarkably, such as 0.2% up 1.8% from gate to die wall, 0.1% up 1.6% from gate to far end.