The interaction between a Slender Body and the Near Wake flow-fields is experimentally and theoretically investigated by the BOS-based free flight. Different analysis techniques allow for the identification of the main structure features. As identified in the near wake flow-fields, the preferred oscillatory motion of the near flow field exhibits two distinct characteristic frequencies, which the most unstable mode pair are still observed. A Fourier decomposition of a BOS slice' divergence is performed to precisely characterise a series of spatial structures information. Then, the input/output behavior of the wake flow is analysis by performing an impedance-matching method of dynamic systems, the equivalent stress–strain relations and energy levels diagram are obtained, which favourably understand and reveal the structures and physical mechanisms of the wake flows. And the aerodynamic parameters identification is also performed, compares with the eigenvalues of the preferred oscillatory motion, indicating that the reciprocal relationship of dynamics characteristics and impedance of wake flow, i.e., there exists a duality between spatial characteristaics of near wake flow-fields and temporal characteristics of the slender body motion.