Valley degree of freedom in the first Brillouin zone of Bloch electrons offers an innovative approach to information storage and quantum computation. Broken inversion symmetry together with the presence of time-reversal symmetry endows Bloch electrons non-zero Berry curvature and orbital magnetic moment, which contribute to the valley Hall effect and optical selection rules in valleytronics. Furthermore, the emerging transition metal dichalcogenides (TMDs) materials naturally become the ideal candidates for valleytronics research attributable to their novel structural, photonic and electronic properties, especially the direct bandgap and broken inversion symmetry in the monolayer. However, the mechanism of inter-valley relaxation remains ambiguous and the complicated manipulation of valley predominantly incumbers the realization of valleytronic devices. In this review, we systematically demonstrate the fundamental properties and tuning strategies (optical, electrical, magnetic and mechanical tuning) of valley degree of freedom, summarize the recent progress of TMD-based valleytronic devices. We also highlight the conclusion of present challenges as well as the perspective on the further investigations in valleytronics.