In the plain of Manasi River Basin, chemical evolution is complex comparatively. From upstream to downstream area, the variation characteristics are mainly that the content of Na+ , Ca2+, HCO3 , SO42- and the value of TDS change between high and low by turns. The water-rock interaction simulation has been applied to study the mechanism of it, the simulation results indicate as followes: (1) Dissolution/precipitation of minerals, evaporation and dilution control the chemical evolution of groundwater jointly; (2) In study area, from upstream area to downstream area, because lithology granularity becomes fine gradually, the dissolution/ precipitation amount of minerals in groundwater increases gently, the water-rock interaction becomes intense accordingly; (3) The water-rock interaction in upstream area undergoes the influence of surface water or irrigation water recharge chiefly, and in middle reaches and downstream area undergoes the influence of evaporation mainly; (4) Because of the large recharge of surface water or irrigation water, the chemical evolution in some areas sometimes doesn't accord with general orderliness, even changes into reverse direction, TDS Value becomes lower, and produces complex middle chemical types.