Solar to hydrogen (H₂) conversion systems based on carbon nanomaterials have shown great potentials in the clean energy field recently. However, for most systems, energy level alignments and light-induced redox processes are still unclear, which hinder artificial designing for higher efficiency of solar energy conversion and further applications. Here we report 77% enhancement in the light-driven H₂ generation efficiency of N,S co-doped carbon quantum dot (N,S-CQD) aqueous system by adding TiO₂ nanoparticles. Using steady-state and transient spectroscopy, four specific energy levels of CQDs are confirmed with the band gaps of 3.55 eV (X₄), 2.99 eV (X₃), 2.76 eV (X₂) and 1.75 eV (X₁), respectively. The X₂ energy band is highly active for H⁺ reduction with a longer lifetime of 13.38 ns. Moreover, the observed low efficiency of intrinsic transition from X₃ to X₂ band of N,S-CQDs accounts for the poor performance of solar to H₂ conversion for pure N,S-CQDs based on H₂ generation and detailed time-resolved spectroscopic results. The mechanism of H₂ generation enhancement can be explained by multiple electron transfer processes between N,S-CQDs and TiO₂ NPs where TiO₂ NPs act as electron intermediates that efficiently transfer electrons from the inert band (X₃) to the active band (X₂) for H₂ generation. This study enriches the fundamental understanding of N,S-CQDs and provides a new pathway toward high-performance N,S-CQD-based solar to H₂ conversion systems.