Phase transition from the semiconducting hexagonal (2H) phase to the metallic monoclinic (1T') phase in two-dimensional (2D) transition metal dichalcogenides like MoTe2 is not only of great importance in fundamental study but also of technological significance for broad device applications. Here we report a universal, facile, scalable and reversible phase engineering technique (between 2H and 1T' phases) for both monolayer and few-layer MoTe2 based on a soft hydrogen plasma treatment. The 2H → 1T' transition was confirmed by a series of characterizations including Raman spectra and mapping studies, XPS analysis and FET device measurements at varying temperatures. We attribute the phase transition to the warping of Te-Mo bonds and the lateral sliding of the top Te-layer induced by the soft hydrogen ion bombardment according to both the structural and electronic characterizations as well as the horizontal comparison with the cases of Ar or O2 plasma treatment. We have also prepared a 2D heterostructure containing periodical 2H and 1T' MoTe2 and showed that such phase transition can be readily reversed by post annealing. These results thus provide a robust and efficient approach for the phase engineering of monolayer and few-layer MoTe2 and could aid the development of 2D optoelectronic, memory and reconfigurable devices.