In compression molding development using the glass fiber mat thermoplastics (GMT) material, CAE is not mature enough yet due to the rheological properties of the GMT material are not measured properly. In this study, we have proposed a method to measure the rheological properties of GMT material through a compression system. Specifically, we have focused on the viscosity of the squeeze flow under the operation of compression processing. The analyzed data is further used for estimation of the rheological parameters and calculation of viscosity at various temperature settings. Moreover, the estimated rheological parameter of the GMT material is integrated into Moldex3D to evaluate the squeeze flow behavior under the compression operation through numerical simulation and experimental study. Results showed that the numerical prediction of the loading force is consistent with that of experimental result. Also, there are some more conditions need to consider in the coming future.