A preliminary study is conducted on the causes of the precipitation anomaly event occurred between January and February of 2009 by using the observational and NCEP reanalysis data．The results show that during this period the height anomalies of 500 hPa at south of 45°N were positive while negative at north of 45°N, which induced more precipitation in the northeast part of Inner Mongolia and less precipitation on other parts; in the central part of Inner Mongolia, both humidity and ascending motion were not favorable for precipitation, and therefore there occurred a severe drought; in the south to east part, ascending motion was favorable for precipitation, but the humidity was not, resulting in drought owing to the dominant humidity; for the north to east part, humidity anomaly was not significant, but ascending motion was favorable for precipitation, resulting in more precipitation.