The phenomenology of dark sector is complicated if dark sector is charged under a confined hidden gauge group. In such kind of model, a dark parton produced at a high energy collider showers and hadronize to a cluster of dark mesons. Dark mesons then decay to visible particles and produce a jet-like signal, which is called "dark jet" in this work. Collider signal of dark jet depends on the property of dark mesons. For example, a finite lifetime of dark meson would provide displaced vertex or displaced track, thus one can use these displaced objects to tag dark jet. However if the lifetime of dark meson is collider-negligible (too short to manifest a displaced vertex), it would be difficult to distinguish a dark jet from SM QCD jets. In this work we propose a new tagging strategy to identify dark jets from QCD backgrounds. This strategy is based on jet-substructure analysis. We study various jet-substructure variables and find out variables with good discrimination ability. Our result shows that by combining multiple jet-substructure variables, one could distinguish dark jets from QCD background, and thus enhance the sensitivity of dark sector search at collider.

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