It is long time to apply hydrologic statistics in series extension. The method to be used is a linear regression model. However, the error of liner regression model is not only concerened with the length of synchronous series and square sum of residue difference, but also increases with distance between independent variable to its mean. Generally, the regression model is not applied to series extension. The weighted regression model is established based on linear regression model with membership function as weight and its unbiasedness being proved. The model has used for series extension in ZIJINGUAN basin and yields satisfactory results.