In this paper, the properties of soliton structures in a hot, unmagnetized dusty plasma which consists of a negatively charged hot dust fluid with variable dust charge, non-thermal distribution of ions and Boltzmann distribution of electrons have been studied. A modified KdV (mKdV) equation which describes the dust acoustic waves in this system is derived by the reductive perturbation method. The effects of dusty plasma parameters on the dust-acoustic solitary waves is studied theoretically. It is found that the soliton structures is determined by the dusty plasma parameters. Furthermore, the soliton only exists in certain regions where these parameters are specially chosen.