Abstract: To address response mismatch in conventional SDOF models, where fixed deformation functions lead to constant transformation factors that cannot capture the time-varying nonlinear response of structures under blast loading, a time-varying-parameter equivalent single-degree-of-freedom model (TVP-SDOF) was proposed. The transformation factors were identified from finite element displacement and energy responses rather than kept constant, enabling the model to track the complete impact response. A fixed-fixed beam under a triangular pulse was used for validation. More crucially, the peak displacement and energy errors were only 3.28% and 4.35%, respectively. During response evolution, the average energy error decreased from 33.4% to 1.1%, and the average displacement error from 16.5% to 1.5%. The results show that the model accurately predicts peak responses, captures pre-peak evolution more reliably, reconstructs response histories well under short intense pulses, and provides a reference for simplified dynamic analysis under continuous impact and sequential blast loading.