Abstract: Based on the moisture static energy (MSE) which has clear physical meanings and is easier to calculate, the causes for the westward shift of interface between Indian summer monsoon and East Asian summer monsoon (IIE) after 2010 were analyzed in this study using National Centers for Environmental Prediction-Department of Energy reanalysis II. The results show that the summer long-term-mean MSE around 90°–110°E，15°–35°N is characterized by saddle-shape. The IIE, defined by the partial deviation of MSE with respect to longitude being equal to zero, is located near 100°E. The evolution trend of the IIE is not obvious in 1979–2010, but significantly move westward after 2010. The intensity contrast between Indian summer monsoon and East Asian summer monsoon is the main causes leading the westward shift of the IIE after 2010. The IIE usually moves toward the relatively stronger one in the Indian summer monsoon and East Asian summer monsoon. In 2011–2024, the intensity of Indian summer monsoon tends to increase, but the intensity of East Asian summer monsoon slightly decreases. The seesaw covariation between the two monsoon systems directly results in the westward shift of the IIE after 2010. Interdecadal Pacific Oscillation is one of the most important external thermal forcing factors leading to the westward shift of the IIE after 2010.