Black phosphorus (BP) has recently emerged as a promising photocatalyst for solar H-2 generation, whereas its bulk crystal with a bandgap of 0.3 eV restricts the driving force for achieving efficient photocatalytic activity. Here, various-sized BP samples were prepared to investigate the role of bandgap in their photocatalytic activities. The results show that the photocatalytic performance of BP samples increases with the decreased layer numbers in the presence of MoS2 as a cocatalyst, which is related to the more sufficient driving force of fewer layered BP nanosheets. Furthermore, large and intimate 2D nanojunctions formed in this smart 2D-2D BP/MoS2 can accelerate the photogenerated charge separation, resulting in the high photocatalytic H-2 production activity. As expected, the 10% BP-10000 nanosheets/MoS2 photocatalyst exhibits the highest H-2 evolution rate of 1286 mu mol.h(-1).g(-1) under visible light irradiation (lambda > 420 nm) with an apparent quantum yield of 1.2% at 420 nm, which is much higher than optimized Pt-loaded BP-10000 sample. Moreover, a considerable H-2 generation rate of 341 is observed under irradiation of the 10% BP-10000 nanosheets/MoS2 photo catalyst with > 550 nm light. It is further anticipated that this study could provide further insight into the design of efficient BP-based photocatalysts, thus offering extendable availability for the construction of noble-metal-free and visible/near-infrared responsive solar H-2 generation system.