Earth's climate system has experienced substantial millennial‐scale oscillations characterized by abrupt changes superimposed on orbital‐scale changes. However, the dynamic connection between millennial and orbital timescales remains elusive. Here, we provide a summer monsoon precipitation δ¹⁸O record spanning from ∼8 to ∼60.5 ka based on the oxygen isotopic composition of microcodium from loess deposits on the Chinese Loess Plateau. This record shows distinct precessional variations (23 ka) punctuated by prominent millennial‐scale oscillations. It is generally consistent with speleothem δ¹⁸O and other loess‐based records, reflecting the East Asian summer monsoon changes. More importantly, our result also shows that large increases in the magnitude of millennial‐scale variability correspond to the Northern Hemisphere summer insolation minimum. We propose that precession‐induced low‐latitude summer insolation modulates the amplitudes of millennial‐scale summer monsoon variability by changing the mean monsoon state during the last glaciation.