The thermal evaporation of Cu₂ZnSnS₄ (CZTS) nanoparticles synthesized by changing the sulfur ratio in the precursor solution was used to investigate the critical role of sulfur concentration in the photoelectrochemical (PEC) water splitting reactions of the CZTS photocathode. X-ray diffraction, transmission electron microscopy, UV–visible spectroscopy, impedance spectroscopy, capacity measurement, and PEC performance measurement were used to investigate the effect of precursor sulfur concentration on the structural, morphological, optical, and PEC performance of films. The half-cell solar-to-hydrogen efficiency of CZTS photoelectrodes increased as the sulfur content increased. The power conversion efficiency of the CZTS cathode constructed with CZTS nanoparticles synthesized with a 0.8 to 1.0 M precursor sulfur concentration was 0.32% at 0 V versus RHE under AM 1.5 illumination. The increase in crystallite size and charge carrier concentration is observed with increasing precursor sulfur concentration, and the maximum crystallite size and charge carrier concentration are observed at 1.0 M sulfur concentration. By carefully adjusting the crystallite size and thus the charge carrier concentration of the CZTS material, the PEC water splitting efficiency of the CZTS photocathode can be improved.