We synthesized transferrable and transparent anion-engineered molybdenum disulfide thin-film catalysts through a simple thermolysis method by using [(NH₄)₂MoS₄] solution and powder precursors with different sulphur/phosphorus weight ratios. The synthesized sulphur-doped molybdenum phosphide (S:MoP) thin film changed from a two-dimensional van der Waals structure to a three-dimensional hexagonal structure by introduction of phosphorus atoms in the MoS₂ thin film. The S:MoP/p-Si heterojunction photocathodes showed a high photocurrent density (33.13 mA/cm² at 0 V vs. the RHE), large overpotential shift (0.9 V at 10 mA/cm²), and long-term stability (over 10000 s). The characterization of the S:MoP/p-Si photocathodes revealed that they could act not only as surface active catalysts owing to their low hydrogen adsorption Gibbs free energy but also as passivation layers owing to their 3D tight bonding between molybdenum and sulphur or phosphorus atoms. Anion-engineering of the MoS₂ thin film catalyst would be an efficient way to enhance the catalytic activity for photoelectrochemical water splitting.