The influence of strain on the behaviors of gas molecules over surface is always limited by the relative weak gas adsorption, the small range of elastic strain of adsorbents and the uniform response of surface sites. However, the latter two factors may be overcome in two-dimensional (2D) materials with defects. In this work, we employ first-principles calculations to investigate the behavior of a series of common gas molecules (CO, CO2, NH3, SO2, NO, NO2 and O-2) on defective (S vacancies and non-metal C/N/O doped) MoS2 monolayers in both the 2H and 1T ' phases under biaxial strain (+/- 5%). When defects are introduced, strain can cause the gas molecules around the defects to physically/chemically adsorb, desorb, dissociate or even react with dopant. Meanwhile, the mechanical energy consumed to generate strain can be effectively transferred to change the energy of adsorption/desorption/reaction of adsorbates with an efficiency of at least 15% (if only adsorption strength is altered) up to 200% (if reaction is triggered). Subsequently, a number of interesting phenomena can be observed. For example, the doping-and-undoping cycle (e.g., the C doping of 2H-MoS2 and N doping of 1T '-MoS2 monolayers) can be dynamically controlled by pumping relevant gases and applying proper strain. Reversible adsorption and desorption of NH3 on defective MoS2 monolayers in both phases can be achieved within +/- 3% strain. NO or NO2 and CO can be converted into non-toxic N2O and CO2 over the N-doped (3% strain) and O-doped (- 3% strain) 1T '-MoS2 monolayers. In essence, defective 2D materials can serve as ideal multi-purpose platforms for strain engineering the behaviors of adsorbates.