Understanding the structural features and the dynamics and properties of charge carriers in photocatalysts is critical to develop them for practical applications. Photocatalytic H2 production on molybdenum sulfide/cadmium sulfide (MoS2/CdS) nanorods in the presence of lactic acid under visible light (λ > 420 nm) was investigated. The optimized MoS2/CdS photocatalysts with 1.52 wt% MoS2 showed the highest rate of 154.748 μmol h−1 mg−1, which is 5 times faster than that of bare CdS nanorods. Experimental results from HR-TEM, UV-vis, and photoelectrochemical measurements suggest that an intimate contact interface, extended light response range, effective separation of the photogenerated charge carriers and high photocurrent density on the MoS2 modification contributed to the photocatalytic enhancement of the MoS2/CdS photocatalysts. Electrochemical measurements indicate that MoS2 is an efficient H2 evolution co-catalyst, which is attributed to the promotion of the photocatalytic activity. Femtosecond transient absorption (fs-TA) spectroscopy was performed to investigate the dynamics of the charge carriers that led to hydrogen production by these composites. The results reveal that the enhanced hole trapping process and effective electrons transfer (within 14.8 ps) from CdS to MoS2 in MoS2/CdS composites can promote their photocatalytic activity dramatically.