Tha Nansihu Lake has been strongly influenced by both of the Yellow River's overflow and human activities.Its forming age has not been determined for a long time.By the two sedimentary cores in the Nansihu Lake, this paper analyzed the environmenntal indexes of pigment, organic δ¹³C and C/N.The ages of the low parts in the cores are measured by ¹⁴C and sedimentation rate of the cores are determined by both of the ²¹⁰Pb and ¹³⁷Cs.Combined with the distribution of pigment, C/N and organic S13C, it can be determined that the forming age of the Nansihu lake is 2450aBP.