Let \(\mathcal {D}\) be a nontrivial Steiner design admitting a group G of automorphisms. In this paper, we study (G, s)-flag-transitivity of Steiner designs. We firstly present a complete classification of (G, s)-flag-transitive Steiner t-designs for \(1\le s\le t\le 5\) . Then we construct some new families of semisymmetric graphs from (G, s)-flag-transitive Steiner designs.