Favourable band alignment and excellent visible light response are vital for photochemical water split-ting.In this work,we have theoretically investigated how ferroelectric polarization and its reversibility in direction can be utilized to modulate the band alignment and optical absorption properties.For this objective,2D van der Waals heterostructures(HTSs)are constructed by interfacing monolayer MoS2 with ferroelectric In2Se3.We find the switch of polarization direction has dramatically changed the band alignment,thus facilitating different type of reactions.In In2Se3/MoS2/In2Se3 heterostructures,one polarization direction supports hydrogen evolution reaction and another polarization direction can favour oxygen evolution reaction.These can be used to create tuneable photocatalyst materials where water reduction reactions can be selectively controlled by polarization switching.The modulation of band alignment is attributed to the shift of reaction potential caused by spontaneous polarization.Additionally,the formed type-Ⅱ van der Waals HTSs also significantly improve charge separation and enhance the optical absorption in the visible and infrared regions.Our results pave a way in the design of van der Waals HTSs for water splitting using ferroelectric materials.