With their high toxicity and fast diffusion, toxic agents such as mustard gas and sarin are chemical warfare agents that are of high lethality and difficult to protect against. Therefore the high-sensitivity detection of toxic agents has become a focus in research on chemical detection in the world. Two-dimensional (2D) MoS ₂is at the forefront of research because of its unique structure and promising sensing performance. In this study, theoretical calculations based on the first-principles method are carried out to investigate the structural stability, electronic properties, and gas adsorption of 2D MoS ₂before and after V doping in order to explain the gas-sensing mechanism of V-doped 2D MoS ₂. The binding energy of V atom at the S-vacancy is –6.85 eV, indicating that the V atom can be stably doped into the S vacancy of the 2D MoS ₂supercell structure at room temperature due to the strong interaction between the doped V atom and S vacancy of monolayer MoS ₂. The V atom doped into the 2D MoS ₂system gives out electrons to surrounding Mo atoms as a donor center, thus enhancing the electric conductivity of the material. The calculation of adsorption energy indicates that the adsorption process of NO ₂, NH ₃, sarin, and mustard gas on the surface of 2D MoS ₂are all spontaneous exothermic reactions. The doping of V increases the adsorption capacity of 2D MoS ₂for the 4 aforesaid gases, and strengthens the interaction between the electrons of the absorbate molecules and those of substrate surface, thus effectively enhancing the gas-sensitive property of 2D MoS ₂. This effect occurs due to the strong overlap between the V 3d orbitals and gas molecule orbitals, which promotes the activation of the adsorbed gas molecules. The analysis of Bader charge shows that the charge transfer occurs from V-doped monolayer MoS ₂to the oxidizing gas molecules (NO ₂, sarin, and mustard gas) acting as acceptors. Whereas the direction of charge transfers is reversed for the adsorption of the reducing gas (NH ₃) behaving as donors, in which 0.11 etransfer from adsorbed gas to metal V-doped monolayer MoS ₂. Our results suggest that V-doped monolayer MoS ₂is an ideal candidate for low-cost, highly active, and stable gas sensors, which provides an avenue to the design of high active 2D MoS ₂-based gas sensors.