The general differences of the geopotential height, temperature and w ind fields between the NCEPFNL and the T213L31 analysis datasets from May 11 t o June 10,2005 are analyzed. The results indicate that the biases at lower laye r of atmosphere are minimum and the maxima often appear in 3 key regions. By usi ng a correlation analysis of biases of terrain height and geopotential height, t he bias of terrain height over the QinghaiTibetan Plateau is regarded as the m ajor force to form the bias of geopotential height between two datasets. Besides , the impacts of the biases on atmospheric circulation at middle and lower layer s are compared and illustrated. The results indicate that the Southwest Vortex a nd the heat source of the plateau in former dataset are stronger than that of th e latter and the shear of wind speed is weak.