The average bias of T639 model forecast fields for 1-10 d forecast in 2009-2010 was evaluated. Forecast fields were included 500 hPa geopotential height, 850 hPa temperature and 2 m temperature. The result showed that T639 model has obviously forecast system errors for the three fields. The decaying averaging method was trying to correct forecast system errors. Corrected result showed that the decaying averaging method has positive correction skill, but correction skill was decreased with lead time. System errors of East Asia were less than the Northern Hemisphere and the correction skill of decaying averaging method for East Asia was also less than the Northern Hemisphere. Through evaluating correction skill of cold season and warm season respectively it is found that the correction skill has no obvious difference for 500 hPa geopotential height and 850 hPa temperature, but for 2 m temperature, the correction skill of cold season was higher than warm season. In addition, the test on different weight of “decaying averaging method” showed that the correction skill was best when weight was round about 0.1 for 850 hPa temperature and 2 m temperature and round about 0.06 for 500 hPa geopotential height.