Two-dimensional (2D) materials play a crucial role in photocatalysis due to their excellent electrical and optical properties. Herein, thickness-controlled CdS nanosheets were synthesized through a simple and low-cost oil-bath method. Sodium citrate, as the structure-directing agent, is paramount in systematically tailoring the thickness and an optimized growth condition in crucial. In particular, the thinnest CdS nanosheets with 1.5-nm-thickness can be obtained when the molar ratio of cadmium source to sodium citrate is 3:5. Moreover, there is a strong correlation between the thickness and photocatalytic hydrogen generation activity of CdS nanosheets. Thinner nanosheets exhibited higher photocatalytic activity, which can be ascribed to the shorter charge transport distance, more active sites and sufficiently negative conduction band edge. A few nanometer difference in thickness has a critical effect on the photocatalytic performance. Meanwhile, the thinnest sample exhibits the optimal performance of photocatalytic hydrogen production (2155 mu mol g(-1) h(-1)), which is about 3.7 times than that of CdS nanoparticles (582 mu mol g(-1) h(-1)). This work highlights the influence of CdS nanosheet thickness on photocatalytic hydrogen generation activity and provides new inspiration for the design of high-efficiency 2D photocatalysts.