The pedocomplex designated as S1 in the western part of the Chinese Loess Plateau, corresponding to MIS 5, has been investigated with the focus on the S1S3 unit (equivalent to Eemian in Europe). The high-resolution data (2-cm interval, ca. 150 years) of the particle size distribution and the frequency-dependent magnetic susceptibility were used to reconstruct variations in the winter and summer monsoon intensities. The data show that the summer monsoon was strongest and the winter monsoon weakest in the late part of the Eemian interglacial. Contrary to previous reports from the central part of the Chinese Loess Plateau, our results show that the East Asian monsoons in the western part of the Plateau were quite stable during the last interglacial. However, our data do exhibit four abrupt events of intensified winter monsoon during the transition from the penultimate glacial to the last interglacial. Also found were the existence of a time lag between the summer and winter monsoons. That is, the intensity of the winter monsoon started to decline prior to the sudden strengthening of the summer monsoon during the transition from MIS 6 to MIS 5e and it gradually returned to a glacial state after the intensity of the summer monsoon rapidly resumed its glacial state from MIS 5e to MIS 5d. The time lag between the two monsoons might have resulted from the different sensitivity of the two indirect driving forces (ice–snow coverage in high latitudes and sea-surface temperature in low latitudes) to global insolation as the ultimate direct forcing factor. Specifically, the winter monsoon had followed the insolation variations very closely, whereas the summer monsoon had delayed to respond to the insolation variations because of the buffering effect of lower latitude oceans.