Photocatalytic water-splitting for hydrogen generation is a promising way to solve the energy crisis, yet the design of efficient photocatalysts is still a challenge. By utilization of first principles calculations, we predict the photocatalytic properties of monolayer boron phosphide (BP) based BP/XY2 (X = Mo, W; Y = S, Se) composites of different rotated configurations. Our results suggest that the BP/XY2 composites can be stably formed, and the narrowed bandgaps ensure these composites are suitable for absorbing visible light. The bandgaps and band edge positions are slightly affected by the rotation angles. The BP/MoS2, BP/MoSe2, and BP/WSe2 are type II heterostructures. Furthermore, the transferred charge from BP to XY2 layers leads to the formation of electric fields, which efficiently separate the photoinduced carriers. The band alignments of BP/MoS2, BP/MoS2, BP/MoSe2, and BP/WSe2 satisfy the requirements of overall water-splitting within the pH scope of 3.6-7.9, 6.8-7.9, 4.0-8.0, and 8.7-8.8. This work will provide valuable insight for designing efficient water-splitting photocatalysts.