| Based on the three-pattern decomposition of global atmospheric circulation (TPDGAC), this study investigates the double-layer structure of the Hadley circulation (HC) and its interdecadal evolution characteristics by using monthly horizontal wind field from NCEP/NCAR reanalysis data from 1948-2011. The following major conclusions are drawn: First, the double-layer structure of the HC is an objective fact, and it constantly exists in April, May, June, October and November in the Southern Hemisphere. Second, the double-layer structure is more obvious in the Southern than in the Northern Hemisphere. Since the double-layer structure is sloped in the vertical direction, it should be taken into consideration when analyzing the variations of the strength and location of the center of the HC. Third, the strength of the double-layer structure of the HC in the Southern Hemisphere consistently exhibits decadal variations with a strong, weak and strong pattern in all five months (April, May, June, October, November), with cycles of 20-30 a and 40-60 a. Fourth, the center of the HC (mean position of the double-layer structure) in the Southern Hemisphere consistently and remarkably shifts southward in all the five months. The net poleward shifts over the 64 years are 5.18°, 2.11°, 2.50°, 1.79° and 5.76° for the five respective months, with a mean shift of 3.47°.