Low-cost and efficient noble-metal-free cocatalysts on semiconductors are highly desirable for visible-light photocatalytic hydrogen evolution reaction (HER). Herein, we report the synthesis and characterizations of novel and high-efficiency noble-metal-free multiphasic MoSe2 nanosheets composed of metallic IT phase and semiconducting 2H phase MoSe2 (denoted as 1T/2H MoSe2), and their implementation as cocatalysts to form photocatalytic heterostructure with 1D CdS nanorods (denoted as 1T/2H-MC). For comparison, conventional 2H MoSe2 and corresponding heterostructure 2H MoSe2/CdS i.e. 2H-MC were synthesized in parallel. The spectroscopic/electrochemical assays show that the 1 T/2H MoSe2 cocatalyst exhibits narrower bandgap, more active sites and better conductivity with respect to 2H MoSe2. As a consequence, the optimized 1T/2H-MC-10% exhibits drastically enhanced visible-light (lambda > 420 nm) photocatalytic HER rate of 93.4 mmol/g/h, obviously higher than those of bare CdS (10.8 mmol/g/h), 2H-MC-10% (30.4 mmol/g/h) and Pt/CdS (50.4 mmol/g/h), respectively. Such an H-2 production rate also exceeds all reported MoSe2-based heterostructure photocatalysts. Most importantly, the multiphasic 1 T/2H MoSe2 remained stable after aging for several months and the HER activity of the 1 T/2H-MC-10% was demonstrated to have no decay even after 8 cycling of visible-light HER.