Molybdenum disulfide (MoS2) are proven to be a promising non-precious-metal cocatalyst for the photocatalytic hydrogen evolution reaction (HER), but the catalytic efficiency of reported MoS2 is still poor due to its limited active sites and low conductivity. Herein, we report a facile one-step hydrothermal approach for synthesizing water-dispersed high-percentage metallic 1T-phase MoS2 quantum dots at a reaction temperature of 180 degrees C (denoted as 1T-MoS2 QDs-180). Such prepared 1T-MoS2 QDs-180 possesses well-dispersed ultrasmall size of similar to 3.3 nm and 1T-phase composition over 82%. Benefiting from the abundance of exposed catalytic edge sites, as well as the excellent intrinsic conductivity of 1T-MoS2 QDs-180, the preformed heterostructure photocatalyst i.e. 1T-MoS2 QDs-180 loading onto CdS nanorods (denoted as 1T-MoS2-C) exhibits remarkably enhanced visible-light (lambda > 420 nm) photocatalytic HER in comparison with pure CdS. Particularly, the HER rate of the optimized 3 wt.% 1T-MoS2-C reaches climbing up to 131.7 mmol h(-1) g(-1), over 65-fold the rate of pure CdS (2.0 mmol h(-1) g(-1)) and appropriately 2-fold the rate of precious-metal Pt loaded CdS (67.0 mmol h(-1) g(-1)). To the best of our knowledge, our home-made 1T-MoS2-C photocatalyst shows the highest visible-light-driven HER performance among all reported phase-engineered MoS2 photocatalytic systems. Such a simple and propagable hydrothermal approach capable of achieving the ultrasmall-sized metallic phase transition metal dichalcogenides is applicable in green hydrogen energy production.