Abstract: Photocatalytic technology, due to its green and environmental friendliness, has a certain application prospect in the fields of hydrogen energy development, pollution purification and medical care in recent years. To explore the influence of the cocatalyst on the performance of hydrogen production, the crystal structure of the cocatalyst itself is studied. The phase structure was successfully transformed by controlling the calcination conditions, and two different crystal phase structures of cobalt selenide (CoSe₂) were prepared, namely orthogonal cobalt selenide (o-CoSe₂) and cubic cobalt selnide (c-CoSe₂). The semiconductor CdS semiconductor was chosen for recombination, and found that the two promoters both have a good promoting effect on the photocatalytic hydrogen production. Through Motschottky curve (MS), UV-vis diffuse reflectance spectra (UV-vis DRS), fluorescence (PL) and photoelectric performance characterization, c-CoSe₂ has stronger conductivity and more efficient charge transport ability than o-CoSe₂, which is theoretically more conducive to the photocatalytic reaction. With lactic acid as the sacrificial agent, the optimal loading of 10wt%o-CoSe₂/CdS and 10wt%c-CoSe₂/CdS hydrogen production efficiencies are 9006.2 μmol·g⁻¹·h⁻¹ and 7151.2 μmol·g⁻¹·h⁻¹, respectively. In general, it has been increased by 20 times and 15 times respectively, which is close to or even surpassing the hydrogen production activity supported by the precious metal platinum (Pt) under the same conditions. The degradation and hydrogen production of 10wt%o-CoSe₂/CdS are achieved in photocatalytic degradation simultaneous with hydrogen evolution. Combining the steps of the photocatalytic reaction and theoretical calculation and analysis, it is found that the more suitable free energy of hydrogen adsorption on the cobalt site of o-CoSe₂ is the key reason for its use as a better hydrogen production co-catalyst.