Abstract: Based on moisture static energy (MSE), which has a clear physical meaning and is easy to calculate, this study analyzed the causes of the westward shift of the interface between the Indian summer monsoon and the East Asian summer monsoon (IIE) after 2010 using the National Centers for Environmental Prediction–Department of Energy Reanalysis II. The results show that the vertical distribution of the summer long-term mean MSE around (15°–35°N, 90°–110°E) exhibits a saddle-shaped pattern. The IIE, defined as the location where the longitudinal derivative of MSE equals zero, is situated near 100°E. Its evolutionary trend was not pronounced during 1979–2010 but shifted significantly westward after 2010. The primary cause of this westward shift is the intensity contrast between the Indian summer monsoon and the East Asian summer monsoon. The IIE typically moves toward the relatively stronger monsoon system. Between 2011 and 2024, the intensity of the Indian summer monsoon tended to increase, whereas that of the East Asian summer monsoon slightly decreased. This seesaw covariation between the two monsoon systems directly resulted in the westward shift of the IIE after 2010. Furthermore, the Interdecadal Pacific Oscillation is identified as one of the most important external thermal forcing factors driving this westward shift.