Phase engineering of two-dimensional transition-metal dichalcogenides (TMDs) has been the subject of considerable interest as it represents a promising strategy for a highly active hydrogen evolution reaction (HER). However, various types of active sites on the basal planes and edges of TMDs have shown complicated mechanisms of the HER in TMDs, hindering the systematic engineering of the catalytic activity of TMDs. Here, we report the intrinsic basal-plane activity of a series of TMDs, Mo1-xWxTe2, whose phases can be engineered from semiconducting to metallic states by adjusting the stoichiometric ratio of tungsten atoms (x). Three forms of 2H- (semiconducting) and 1T′-(metallic) Mo1-xWxTe2, bulk, powder, and exfoliated flakes, as well as microreactors, were used to investigate the HER process of the phase-engineered TMDs. The catalytic activity of Mo1-xWxTe2 exhibits the best performance at the phase-transition boundary (i.e., x = 0.09) with a hydrogen conversion rate of 0.692 s-1, which is 10-20 times higher than that of other 2H and 1T′ samples with different x values. Our study provides a novel approach, using the phase-transition boundary, to modify the catalytic activity of polymorphic nanomaterials.