Layered transition metal dichalcogenides (TMDs) have attracted increasing attention for semiconducting applications owing to their distinctive properties, such as high surface-to-volume ratio, good electronic transport and phase-engineered. However, most of the methods for fabricating thermodynamic metastable phase have so far been induced by conventional CVD process. The high synthesis temperature may cause damages to the substrate and also the processing time is extended. In this concept, here, we incorporated a plasma function into a typical selenization process, which is then applied to metal oxides, namely, a plasma-assisted chemical vapor reduction (PACVR) process, to overcome the disadvantage of CVD process since the presence of high-energy Se ions under plasma treatment lower the energy barrier for a reaction and facilitate the formation of TMDCs at lower temperature. In this work, we developed a facile method to achieve phase control and report a ppb-level detection gas sensor on formaldehyde. Tunable phase synthesis of WSe₂ can be achieved by varying conditions such as temperature, carrier gas flow rate and plasma power in a low-temperature PACVR system. Through the detailed investigation of chemical bonding in WSe₂ by Raman and XPS, it was found that 100% 2H phase and hybrids of 1T^’ and 2H phases can be achieved, and the ratio of 1T^’/2H is tunable, relying on substrate temperatures under plasma treatment. By investigating the sensing behavior towards HCHO gas, the performance is highly associated to the ratio of 1T^’/2H. The responsivity of 33 % can be achieved with the ratio of the hybrids around 0.46 in WSe₂. The phase-engineered 2D-WSe₂ layers grown by plasma-assisted chemical vapor reaction with tunable performance can be applied to detect other VOCs efficiently.