Abstract: Based on the calculation of wave number of Rossby wave and flux of wave activity, the characteristics of Rossby wave propagation in southern branch of westerlies are analyzed, and the relationship between Rossby wave propagation in westerly jet stream and the formation mechanism of the wintertime southern branch trough in the subtropical westerlies is also investigated by using 58-year monthly and daily NCEP/ NCAR reanalysis data with the aid of one point correlation, EOF, harmonic and composition analysis.Results show that the reare three westerly disturbances under the subtropical westerly jet stream in Arabian Sea, the Bay of Bengal, and South China of Northern Hemisphere during the winter half year. Southern branch trough over the Bay of Bengal is a semi-permanent trough due to the smallest variability. A teleconnection wave train with negative, positive and negative centers, migrating from North Africa to the Bay of Bengal via Arabian Sea, suggests that southern branch trough is positively related with the trough over North Africa.In the propagation process from North Africa to the Bay of Bengal, the westerly disturbances usually pause or strengthen in the Arabian Sea.The propagation from west to east takes about 20 days, appearing remarkable quasi-biweekly oscillation.There are waveguide regions of wave number 6—8 along the subtropical westerly jet stream in the half year of winter.Because jet stream waveguide is the strongest and the stationary Rossby wave energy in the lower troposphere coming from North Africa is transported to the Bay of Bengal, southern branch trough becomes deepest in February.Bay of Bengal is also a main source place of southern branch trough in the winter half year, and the development of trough over South China mainly connects with the eastern propagation of southern branch trough.Moreover, the activity of the cold air along the eastern and western sides of the the Plateau plays an important role in the formation of southern branch trough.After the breaking down of cold air, it first arrives at Bay of Bengal along the southern sides of the Plateau and then departs from the western sides of the Tibetan Plateau, and makes the southern branch trough gradually deepened.The eastern cold air of the Tibetan Plateau breaks down in the wake of cold surge breaking down in East Asia.Cold air at surface layer from north-east to south-west disperses to the region of India and Burma, which is another important factor of development of southern branch trough.