Photocatalytic water splitting is a promising technology to solve serious energy and environmental problems. The PtS2 monolayer has been previously predicted to be a water splitting photocatalyst. But the high efficiency of carrier recombination in the monolayer results in poor photocatalytic performance. It is well known that the construction of van der Waals (vdW) heterojunctions can improve the photocatalytic performance of a monolayer. In this investigation, we constructed a PtS2/SnS2 vdW heterojunction and systematically investigated the influence of the doping position and doping ratio on its performance using density functional theory calculations. Interestingly, the band alignment transforms from Type-II to Type-I and from Type-I to Type-II when the S in SnS2 is replaced with Se in the PtS2/SnS2 vdW heterojunction and the S in PtS2 is replaced with Se in the PtS2/SnSe2 vdW heterojunction, respectively. More importantly, from the PtS2/SnS2 to PtSe2/SnSe2 vdW heterojunction, the decomposition of water also changes from semi-decomposed water to fully decomposed water. Furthermore, the results show that the direct Z-scheme photocatalytic mechanism exists in the PtSSe/SnSe2 vdW heterojunction by analysis of the migration paths of photoinduced electrons and holes. And compared with the PtS2/SnS2, the PtSSe/SnSe2 heterostructure exhibits better photocatalytic water splitting activities. These results can provide a direction that doping can improve the photocatalytic water splitting performance of heterojunction photocatalysts.