Designing non-precious-metal bifunctional electrocatalysts with highly efficient overall water splitting in basic media still remain a signification challenge. In this work, an environment-friendly and one-step strategy is developed to synthesize two-dimensional (2D/2D) NiS/graphene heterostructure composites via a pyrolysis accompanied sulfidation process using novel deep eutectic solvents (DESs) as precursors. The as-obtained 2D NiS/graphene heterostructures manifest excellent HER and OER activity in alkaline solution. Furthermore, the optimized 2D/2D heterostructure as a bifunctional catalyst for overall water splitting exhibits a low cell voltage of 1.54 Vat 10 mA cm(-2) in alkaline media, even superior than that of the integrated Pt/C and Iro(2) couples. The one-step DESs based procedure favors the interface coupling between graphene and NiS nanosheets, which significantly brings high surface-active sites exposure, enhances electrical conductivity, and results in high electrocatalytic activity. The DESs precursor has the advantage of solution reaction and acts as an environment-friendly synthetic strategy for clean energy generation.