Well-defined ultrathin MoS2 nanoplates are developed by a facile solvent-dependent control route from single-source precursor for the first time. The obtained ultrathin nanoplate with a thickness of ~ 5 nm features high density of basal edges and abundant unsaturated active S atoms. The multistage growth process is investigated and the formation mechanism is proposed. Ultrathin MoS2 nanoplates exhibit an excellent activity for hydrogen evolution reaction (HER) with a small onset potential of 0.09 V, a low Tafel slope of 53 mV dec(-1), and remarkable stability. This work successfully demonstrates that the introduction of unsaturated active S atoms into ultrathin MoS2 nanoplates for enhanced electrocatalytic properties is feasible through a facial one-step solvent control method, and that this may open up a potential way for designing more efficient MoS2-based catalysts for HER.