Cobalt phosphide (CoP_x) has been developed as a cost-effective cocatalyst for photocatalytic H₂ evolution with the advantages of excellent conductivity, strong reduction ability, good thermal and chemical stability. In this work, a facile preparation strategy was proposed to fabricate ultrafine CoP_x nanoparticles on the surface of CdS. The effective H₂ production over ultrafine CoP_x nanoparticles has been realized. In addition, we found Ca²⁺ as an alkaline earth metal ion can promote the interaction between CdS and CoP_x. Both experimental results and density function theory indicate that the Ca²⁺ dopant can act as surface trapping sites on CdS and lead to efficient separation of photogenerated electron-hole pairs. The integration of CoP_x and Ca²⁺ dopant can synergistically enhance both photogenerated electron-hole separation as well as interfacial charge transfer, which enables a remarkable improvement on the H₂ generation performance of CdS. The photocatalytic H₂ generation rate of Ca-modified CoP_x@CdS can reach up to 2441.5 μmol h⁻¹ under optimal conditions with the apparent quantum efficiency as high as 35.4% at 420 nm. This finding motivates the development of simplified fabrication procedures for constructing and modifying cobalt active sites with efficient photocatalytic H₂ generation performance.