Abstract: The composition of groundwater is very complicated. It contains inorganic ions and dissolved organic compounds (DOC).As one aspect of the environmental chemistry of the actinide elements,the influences of inorganic ions,concentration of humic acid,pH value of the solution and temperature, etc. on reduction rate of Pu (Ⅵ) are studied. The results indicate that the reduction rate of Pu (Ⅵ) is proportional to the concentration of Pu (Ⅵ). With the increasing humic acid concentration and the pH value, the reduction of Pu (Ⅵ) accelerates obviously. The rate equation of reduction reaction of Pu (Ⅵ) is; - dPu(Ⅵ)/dt= kPu(Ⅵ)HA0.52H+-0.069The reduction rate of Pu(Ⅵ) increases with the increase of temperature. Because the temperature of deep groundwater is so high, it can be estimated that the reduction of Pu (Ⅵ) would be very fast in deep underground condition.The influences of Na+ ,K+ ,Ca2+ ,Mg2+ ,NO3- ,HCO3- and HPO42- etc. on the reduction of Pu (Ⅵ) are not significant, but the existence of Fe2+ ,F- and SO42- accelerates the reduction of Pu( Ⅵ) obviously.The reduction behaviour of Pu (Ⅵ) in simulated groundwater is plotted. It can be seen that the curves of the Pu (Ⅵ) content against reaction time can be devided into two straight lines. The possible reason of decreasing slope of Pu (Ⅵ) reduction could be explained by the fact that under the open experimental condition,Fe2- could not exist for a long period,thus causing the gradual lowering of the reduction power.