Dynamic buckling and post-buckling properties of simply supported imperfect columns under axial fluid-solid interaction are experimentally and numerically investigated in this study. By observing the elastic-plastic responses of imperfect columns, three critical criteria are defined for the columns with prescribed imperfection, namely, the dynamic buckling criterion, the dynamic yielding criterion and the plastic collapse criterion. The relevant critical impulses are determined for each column. The dynamic buckling and collapse mechanism, and the effects of dynamic load duration and initial imperfection of column on the elastic-plastic dynamic buckling and post-buckling properties are examined. The results indicate that, for columns with small imperfection, dynamic buckling will occur with the increase of the fluid-solid impulse. For columns with large imperfection, however, their failure is dominated by the flexural responses. The dynamic buckling and post-buckling characteristics of columns under fluid-solid interaction are also compared and discussed with respect to those of columns under solid-solid impacts. It is found that they are different from those of columns subjected to either impulsive impact or impact owing to suddenly applied load. (C) 2001 Elsevier Science Ltd. All rights reserved.