Abstract: In this paper a new method is given to judge the stability of the EOF exparaded precision. The results show that:l,The expanded precision of summer(winter) anomaly geopotential height by using the EOFs of winter (summer) anomaly geopotential height is approximately equal to that of winter (summer) anomaly geopotential height by using the EOFs in winter (summer);2,The differences among expanded precision of the anomaly geopotential height in the year and those in other years based on EOFs of the anomaly geopotential height in the correspondent year are small. It means that the expanded precision of the observed data based on the EOFs is stable in the range of the normal variability of external forcing. So it is feasible that a simplified model with the small number of the degrees of freedom can be gotten when the EOFs of the observed data are used as basis.