Twinned Mn_0.5Cd_0.5S (T-MCS) solid solution was prepared by a hydrothermal method, and then Zn_0.76Co_0.24S/T-MCS heterojunction was fabricated by an in-situ hydrothermal method. The results show that Mn_0.5Cd_0.5S solid solutions is a twinned homojunction consisting of wurtzite Mn_0.5Cd_0.5S (WZ-MCS) and zinc-blende Mn_0.5Cd_0.5S (ZB-MCS) alternately. The introduction of Zn_0.76Co_0.24S can enhance the light harvesting ability of the system and increase the number of the surface charge carriers, the H₂ production rate of 3% Zn_0.76Co_0.24S/T-MCS reaches 132.9 mmol·g⁻¹·h⁻¹ in Na₂S/Na₂SO₃ mixture solution (300 W Xe lamp, λ > 420 nm), which is 332.2 and 1.9 times greater than those of Zn_0.76Co_0.24S and T-MCS, respectively. According to the results by energy band structure analysis, the type-II twinned homojunction between WZ-MCS and ZB-MCS can improve the bulk phase charge separation, the S-scheme heterojunction between T-MCS and Zn_0.76Co_0.24S can accelerate the interfacial charge transfer, and the REDOX capacity of the holes in T-MCS valence band and electrons in Zn_0.76Co_0.24S conduction band is retained, therefore resulting in a faster H₂ production.