During May 31 to June 1, 2010, an extraordinary rainstorm process occurring in southern China included three concentrated precipitation periods and approximately four times the normal mesoscale convective system (MCS) activities. Among them, a quasi-stationary training line/adjoining stratiform (TL/AS) MCS was maintained for more than 10 h in central Guangxi Zhuang Autonomous Region, resulting in extreme precipitation events recorded by multiple stations. Using observation data and simulated results, the observation characteristics and continuing environmental conditions of this system were analyzed in this study. The results showed that the quasi-stationary TL/AS MCS was developed in an upper divergence and mid-lower cyclonic circulation environment. The MCSs forming in thermodynamic environments were characterized by very high relative humidity at low levels, moderate convective available potential energy, and very little convective inhibition. The presence of a strong low-level jet (LLJ) and weak midlevel winds led to a pronounced reversal of the wind shear vector with height. The vertical wind shears were largely perpendicular to the convection line at low levels, but were mainly parallel to the line at mid-levels. These wind shear characteristics may have caused the TL/AS MCS to become quasi-stationary. The lifting and destabilization associated with interaction between the LLJ and midlevel circulation assisted in initiating and maintaining the long-lived, slow-moving MCSs.