Earth-abundant two-dimensional (2D) transition metal dichalogenate (TMD) nanosheets manifesting both polymorphic 1T’ metallic and 2H semiconducting characteristics have been demonstrated as efficient photocatalysts. Despite considerable efforts have been devoted to synthesizing such material, it is still a stunning challenge to precisely tailor the ratio of metallic and semiconducting phases within an individual single-layer 2D TMD nanosheet to maximize their potential in photocatalysis. Herein, we developed a chemical alloying strategy through incorporating W and/or Se elements into MoS2, which enables precise tuning the ratio of 1T’ and 2H phases in single-layer TMD nanosheets. Aberration-corrected scanning transmission electron microscopy reveals the presence of the 1T’/2H heterophase in individual single-layer MoxW1-xS2 nanosheets. Density functional theory (DFT) simulations reveal that the tunability of 1T’ and 2H heterophase enabled through W element alloying optimize the balance of charge separation and absorption. This consequently boosts significantly the photocatalytic hydrogen production reaction from a liquid carrier of methanol, yielding a record high hydrogen generation rate of 1,426 mmole·g-1·h-1 in single-layer Mo0.21W0.79S2 nanosheets.