Abstract: The near-field region of an explosion is the core zone of munition-induced damage, characterized by the coupled loading of intense shock waves and detonation products. Currently, the mechanical response and energy conversion mechanisms of Expansion Tube Structures (ETS) under such extreme loading conditions remain unclear. In this study, ETS is adopted as a representative energy-absorbing structure to investigate its energy conversion behavior under the coupled action of near-field shock waves and detonation products. Based on experimental validation, numerical simulations are employed to analyze the characteristics of near-field blast loads and the dynamic response of ETS. Furthermore, a theoretical prediction formula for near-field blast loads is established, and a theoretical model for predicting energy conversion efficiency is developed under the strong-shock assumption. The results show that the energy conversion efficiency decreases significantly with increasing scaled distance, dropping below 10% when the scaled distance exceeds 0.8 m/kg¹/³. Moreover, the energy conversion efficiency exhibits a strong positive correlation with the specific impulse of the reflected wave, indicating that specific impulse is the key factor governing energy transfer. This work elucidates the intrinsic energy conversion mechanism of ETS under near-field coupled loading, and the proposed theoretical model provides a robust foundation for the design and performance evaluation of near-field protective structures.