In the past few years, there has been considerable emphasis on investigating fracture behavior in composite materials using the phase-field fracture (PFF) method. However, accurately characterizing the viscoelasticity of the matrix necessitates employing the viscoelastic PFF method, and there remains an ongoing debate regarding the precise determination of the viscous driving force introduced in this methodology. This study aims to propose a feasible calibration method for quantifying the viscous driving force. To achieve this goal, benchmark experiments utilizing Polyetheretherketone (PEEK), a typical thermoplastic resin, are carried out by designing uniaxial tension tests, three-point bending tests, as well as compression tests at various strain rates. Through employing the viscoelastic PFF method, we discuss the effect of strain rate and viscous driving force on mechanical response. Our findings reveal a non-linear relationship between the influence of viscous driving force and strain rate with an initial increase followed by a subsequent decrease. By combining simulation results with experimental data analysis, the viscous driving force value is determined. The results demonstrate that our proposed approach is reliable for quantifying the viscous driving force in the viscoelastic PFF method, laying a foundation for investigating the mechanical response of composite materials.