Linear decomposition time series analysis has become one of useful tools to investigate runoff characteristics, but how the sequence length affects the results of linear decomposition has not yet been studied in detail. In this paper,the runoff time series at Zhuqi station of the Minjiang River and the inflow time series at Baishan reservoir of Songhuajiang River are selected to investigate the effects of time series length on rurr o ff characteristics. Six sequence lengths are selected according to the available runoff time series over 70 years, and the linear decomposition for different length is conducted. Results show that the characteristics of runoff,such as shift trend,jump change, and periods have been affected obviously by sequence length. It is suggested that the reasonable sequence length necessary to conduct linear decomposition and reasonable sequence length may be correlated with the time scale of time series.