The rational design of a photocatalytic cocatalyst is of significant importance for improving the conversion efficiency of solar energy to hydrogen energy. This work indicated that the band structure and hydrogen evolution reaction overpotential of MoS₂ cocatalyst derived from phosphomolybic acid (PMo₁₂) could be facilely modulated through phosphorus doping. The PMo₁₂-derived MoS₂ could be fabricated at a very low temperature, and exhibited a dramatic enhancement of the photocatalytic hydrogen production activity of CdS. Notably, phosphorus doping over MoS₂ could further improve the photocatalytic activity due to the modulated band structure and electrocatalytic performance. Consequently, the photocatalytic hydrogen evolution rate of the pristine CdS could be raised from 0.31 to 4.59 mmol h⁻¹ g⁻¹ by embedding the PMo₁₂-derived MoS₂ nanosheet, and further to 8.86 mmol h⁻¹ g⁻¹ after phosphorus doping of MoS₂, opening a new method for improving the photocatalytic activity by the rational design of the cocatalyst.