We obtained several displacement time series from the Sichuan permanent GPS net and processed the 1-Hz data observed during a few days before the 2008 Ms8.0 Wenchuan earthquake by double-difference instantaneous positioning technique.We filtered the data by the spatial stacking and the modified sidereal filtering methods to reduce correlation bias in space and time.The results indicate that these methods can improve the precision significantly. We obtained several displacement time series from the Sichuan permanent GPS net and processed the 1-Hz data observed during a few days before the 2008 Ms8.0 Wenchuan earthquake by double-difference instantaneous positioning technique. We filtered the data by the spatial stacking and the modified sidereal filtering methods to reduce correlation bias in space and time. the results that results that methods can improve the precision significantly.