The crystal structure of CdS influences the energy band gap and therefore, its suitability as a semiconductor photocatalyst for solar-to-hydrogen energy conversion. By simply adjusting the temperature of the hydrothermal reaction, CdS samples with different crystal phases were prepared, and their photocatalytic hydrogen production performance was tested to investigate the photocatalytic mechanism of cubic CdS. XRD results confirmed the successful preparation of the two catalysts. TEM results revealed that the (111) and (100) crystal planes corresponded to the cubic and hexagonal CdS structures of the two catalysts. The band gap values of cubic phase and hexagonal phase CdS were 2.24 eV and 2.17 eV, respectively. Electrochemical impedance spectroscopy results showed that cubic CdS exhibits a smaller arc radius and lower resistance. The conduction potential of the two CdS phases was further calculated based on the Mott-Schottky plots, revealing that the conduction potential of cubic CdS is more negative than that of hexagonal CdS. Therefore, cubic CdS exhibited a higher carrier migration rate and charge separation efficiency than hexagonal CdS; the photocatalytic hydrogen production of cubic CdS reached 680 µL, which was considerably higher than that of hexagonal CdS under visible light irradiation. This study may serve as a guide for the development of potential applications of CdS-based materials in the field of visible-light-driven hydrogen evolution and could pave the way for the fabrication of other high-performance CdS/semiconductor composites.