The undeveloped molybdenum borides (Mo2Bx, x = 1 or 4) were investigated as the active ingredients coupled with CdS for photocatalytic hydrogen production for the first time. Display omitted •The undeveloped Mo2Bx were used for photocatalytic HER for the first time.•The CdS-Mo2Bx (15 wt%) exhibit excellent photocatalytic HER activities.•Mo2Bx show strong boron dependence in electrocatalysis and photocatalysis. Molybdenum based compounds, such as MoCx, MoSe2, MoS2 MoPx, MoOx and their derivatives, have been considered as the alternative catalysts for platinum (Pt) whether in electrocatalytic or photocatalytic hydrogen evolution reaction (HER). This means molybdenum based compounds have great application potential in solving the energy crisis and environmental pollution. Herein, for the first time, the unexplored Mo2Bx (x = 1 or 4) were investigated as the active components in a visible-light driven photocatalytic hydrogen production system. Experimental results demonstrate that they both exhibit excellent hydrogen evolution activities when combined with CdS or organic photosensitizers. Additionally, for the same kind of light-harvesting material, the photocatalytic HER activity of Mo2B4 is obviously better than that of Mo2B. This clearly indicates the photocatalytic activities of them are affected by the content of electron deficient atom boron (B) in Mo2Bx samples and exhibits a strong B dependence in photocatalytic HER. The reason for this B dependency is studied in detail by photo/electrochemical methods and optical characterizations in this work, where it is essentially caused by the difference in the electron transfer (ET) rates between light-harvesting material and Mo2Bx samples and just manifested as B dependency. Additionally, the relation between photocatalysis and electrocatalysis in HER is discussed.