Numerical experiments were conducted on a squall line occurring on June 3-4, 2009, in Henan, Anhui, and Jiangsu provinces, China, that produced high winds and hail. The impact of moisture on the entire air column and that of its vertical distribution on the intensity, development process, and morphology of mesoscale convective systems (MCSs) were investigated. Analysis revealed that the amount of moisture and its vertical distribution had a significant effect on the strength of the downdraft and cold pool, which thus affected the morphology, duration, and strength of convection. The experiments on the entire air column demonstrated that the intensity of MCSs increased with moisture and that the strengths of the cold pool and the thunderstorm high increased, which led to higher winds. Conversely, the intensities of MCSs, cold pool, and surface winds decreased with moisture. Additional moisture led to a stronger cold pool, which caused the maximum winds to strengthen at the developmental stages of the MCSs. However, the cold pool and rear inflow jet weakened more rapidly, which was unfavorable for high wind development and maintenance at mature stages. Precipitable water in entire air column remained unchanged. Linear MCSs and high winds tended to occur in environments of mid-level drier air and low-level moister air that favored the development of stronger thunderstorm highs and discouraged the persistence of MCSs.