In this paper, the causes of the continuous drought occurred in Southwest China from October of 2012 to April of 2013 are analyzed with a variety of data by diagnosing the influence of large scale atmospheric circulation, water vapor transportation, tropical sea surface temperature, Arctic Oscillation and stratospheric polar vortex. The results show that the difficult intersection of cold and warm air over the Southwest China caused by the decrease of the southwestern water vapor transportation and the eastward pathway of the cold air from high latitudes was the direct cause of the continuous drought in the Southwest China. The Southwest China was dominated by the straight westerly flow and the geopotential heights of the Bay of Bengal and Indo-China Peninsula were higher than normal during this time. It was not conducive to the development of cyclonic circulation over these regions. Changes of the tropical sea surface temperature especially for the Indian Ocean which was persistently high have enhanced the abnormal low-level anticyclone circulation over the Philippine area during winter and spring. All above these have led to weaken the southwestern water vapor transportation. In addition, the abnormal changes of Arctic Oscillation also played an important role in the continuous drought. The sustained negative phase of Arctic Oscillation not only made the Southern Trough become weaker so that the southwestern water vapor transportation reduces, but also weakened the Lake Baikal ridge so that the pathway of the boreal cold air shifts eastwardly. Changes of the strength of stratospheric polar vortex had a significant effect on the intensity and pathway of the cold air which can be reflected by geopotential height and potential vorticity at different levels in winter.