Simulation based design optimization has become the common practice in automotive product development. Increasing computer models are developed to simulate various dynamic systems. Before applying these models for product development, model validation needs to be conducted to assess their validity. In model validation, for the purpose of obtaining results successfully, it is vital to select or develop appropriate metrics for specific applications. For dynamic systems, one of the key obstacles of model validation is that most of the responses are functional, such as time history curves. This calls for the development of a metric that can evaluate the differences in terms of phase shift, magnitude and shape, which requires information from both time and frequency domain. And by representing time histories in frequency domain, more intuitive information can be obtained, such as magnitude-frequency and phase-frequency characteristics. However, Most of the existing metrics only focus on time domain. In this paper, a validation method combining analysis in time and frequency domain is proposed. Two analytical cases are then utilized for the illustration of the proposed method and process. A real-world children restraint system design case is then conducted. The evaluation results are consistent with Subjective Matter Experts' judgments.