The isotopic technique is used in the study of the evolution of groundwater in the Yongdinghe River Plain in Beijing. The sampling profile, which has fourteen sampling locations, is along the flow direction of the groundwater in the Yongdinghe River pluvial and alluvial fan. The 14C and tritium contents of the groundwater are detected and the ages of the samples are calculated. The 14C age of the superficial porous aquifer ranges from 730a to 4900a, and that of the deep porous aquifer ranges between 13420a and 22480a. The 14C age gradually increases from the superficial to the deep aquifers in a vertical direction,and the largest range is from 3010a to 22480a. The contents of tritium of the superficial aquifer varys from 14.99 to 30.56Tu, and that of the deep aquifer ranges between 0.51 and 4.71Tu. By analyzing the changes in vertical and horizontal directions in the 14C age and the contents of tritium of the groundwater, we can examine the direction of the groundwater flow and calculate the velocity of the flow. The flow velocity ranges from 62.63m/a to 5.02m/a. The flow velocity of the superficial aquifer gradully decreases from the piedmont area to the plain, and subsurface runoff also decreases. The hydologic circle of the superficial aquifer is mainly vertical, and that of the deep aquifer is mainly horizontal.