The rate-dependent effect of viscoelasticity plays a critical role in the hardening mechanisms of impact-hardening polymers (IHP) when forcefully impacted. In this study, we used dynamic mechanical analysis (DMA) to characterize the rate-dependent viscoelasticity of an IHP under oscillatory shear. We found that the storage modulus increased by three orders of magnitude within the experimental range when the oscillatory frequency varied from 0.1 to 100 rad/s. To further understand the real strain rate effect of IHP, we introduced the Havriliak-Negami (H–N) model to predict the dynamic viscoelastic behaviors of the IHP for a wider frequency range (from zero to infinity) than that applied in the DMA experiments. Based on the H–N model results, we defined a parameter to describe the rate-dependent effect of the IHP, which was not dependent on the frequency range and reflected the intrinsic material properties of IHP. We used the time-temperature superposition principle (TTSP), which extended the experimental range from 0.1 rad s−1 down to 0.005 rad s−1, to verify the accuracy of the rate-dependent viscoelasticity predicted by the H–N model. Finally, we outlined the influence of temperature on the dynamic viscoelastic behaviors of IHP and discussed the phase transition mechanism induced by temperature and the oscillatory frequency. The results presented here not only provide a method (i.e., by combining experimental results with the H–N model results) to characterize the real rate-dependent viscoelasticity of IHP but are also valuable to further our understanding of the impact-hardening mechanisms of IHP.