Baesd on Palmer drought index,a flood/drought index series from 1951 to 1996 in the Tianmu Mountain area was established,and this dynamic index can reflect the persistent character of floods/droughts.The tree α-cellulose δ¹³C time series from 1835 to 1997 was obtained based on cross-dating tree rings age from the Tianmu Mountain,Zhejiang Province.After removing the long-term decrease trend,which is related to atmospheric CO₂ concentration of the δ¹³C series,we analyzed the correlation between the δ¹³C and the flood/drought index of different periods.The results suggest that both the same year and the next year tree ring δ¹³C are distinctly related with the spring and summer drought indices,showing that the δ¹³C better responsed to the spring and summer flood/drought.Based this better correlation and using the multiple regression method,we reconstructed the historical spring and summer flood/drought index from 1835 to 1996.The reconstructed series shows obvious decadal variability,and thus provides a reliable reference for the local climatic change study.