Although graphitic carbon nitride (g-C3N4) is one of the most promising metal-free semiconductors in the field of photocatalytic hydrogen production, the preparation of efficient g-C3N4-based photocatalysts is still a challenge. Herein, the strategy of element doping and co-catalyst loading are employed to make an effective modification on g-C3N4. The Mo2C hollow nanospheres supported by porous B-C3N4 (B-doped g-C3N4) flakes, namely, B-C3N4/Mo2C photocatalysts are successfully constructed by the ultrasonic self-assembly-calcination approach. The unique Mo2C hollow nanospheres structures increases internal multiple visible light scattering, which facilitates light-harvesting, shortens the transport distance of carriers, and hence reduces the carriers recombination. Impressively, B-C3N4/Mo2C-35 exhibits excellent activity in photocatalytic hydrogen production, affording an H2 production rate up to 1696.4 µ mol gâ'1 hâ'1, which is higher than B-C3N4/3 wt% Pt photocatalyst. Moreover, the apparent quantum efficiency of B-C3N4/Mo2C-35 at 420 nm is 2.12%. Mechanism studies suggest that this desired photocatalytic performance is attributed to a broader light absorption range, more reactive sites and faster carrier transfer rate than that of pure g-C3N4. This work develops a noble metal-free hollow nanosphere co-catalyst system and proposes new insight into the design of g-C3N4-based composite photocatalysts. [ABSTRACT FROM AUTHOR]