The moist circumstance (e.g., O-2 and H2O molecule) is one of the critical factors affecting the application of MoS2, which lead to MoS2 failure. Fortunately, metal-atoms doping MoS2 is commonly used to decrease damage of MoS2 by O-2 or H2O molecule. To understand and explore the microscopic mechanism of MoS2 failure, the interaction of O-2, H2O molecule with defect-free, defected MoS2 is calculated using first-principles method. It could be found that the failure of MoS2 results mainly from the defects, which make the exposed Mo atoms more active and easy to react with O-2 or H2O molecule. Therein to, S-vacancy defect of MoS2 (MoS2-V-s1 species) is the reaction outset point of O-2 environment, and triple vacancy defect of MoS2 (MoS2-V-s2(+Mo) species) is that of H2O environment. Based on this failure mechanism, the typical Ti or Pb elementals doped MoS2 are studied, the calculation results indicate that Ti or Pb doping elements can retard effectively the failure of MoS2 in humid environment through reacting preferentially with or repulsing to O-2 or H2O molecules, and even restraining the diffusion of H atom dissociated from H2O molecule. All of these will be helpful for the design, synthesis, and application of related transition metal dichalcogenides.