A gradually stepped CdS–Mn/MoS₂/CdTe/TiO₂ quaternary photocatalyst is successfully synthesized by a reliable and effective method. The as-synthesized CdS–Mn/MoS₂/CdTe/TiO₂ photocatalyst showed excellent improvement for visible-light absorption and the separation efficiency of photoinduced electron–hole pairs. The CdS–Mn/MoS₂/CdTe/TiO₂ photoelectrode exhibits optimum visible-light-induced photocatalytic activity in photoelectrochemical (PEC) immunoassay of tris-(2,3-dibromopropyl) isocyanurate (TBC) and producing hydrogen (H₂) out of water, and a linear range of 2 pM to 100 nM, a low detection limit (3 S/N) of 1.02 pM and a H₂ evolution rate of 10.165 mL cm⁻² h⁻¹ are obtained. The excellent photocatalytic activities are attributed to the synergetic effects of CdS–Mn, MoS₂, CdTe and TiO₂ nanophase structures in the CdS–Mn/MoS₂/CdTe/TiO₂ composites, which result in a step-wise-mechanism-assisted fast separation and slow recombination of photoinduced electron–hole pairs and thereby higher photocatalytic activity. The electrochemical impedance spectra and the significant enhancement of photocurrent responses indicated that the stepwise structure of band-edge levels dramatically facilitated the electron transfer in the CdS–Mn/MoS₂/CdTe/TiO₂ nanocomposite electrode. The CdS–Mn/MoS₂/CdTe/TiO₂ photoelectrode was demonstrated to be effective for photoelectrochemical sensing of trace environmental pollutants in water samples and solar H₂ generation out of water.