The relative influence of insolation, CO₂, and ice sheets on the East Asian summer monsoon (EASM) is not well understood especially at regional scale. Here a Gaussian emulator based on simulations with HadCM3 is used to quantitatively assess how astronomical forcing, CO₂, and northern hemisphere ice sheets affect the variation of the summer precipitation over the last 800 ky. Our results show that in the EASM domain north of 25°N, the variation of the summer precipitation is dominated by precession, leading to strong 23-ky cycles, while the ice sheets only modulate the effect of insolation by influencing the land-sea pressure gradient. In the southern part, ice sheets play a more important role, generating 100-ky cycles, through influencing the latitude of the Intertropical Convergence Zone and the Hadley cell. Obliquity and CO₂ have little effect on the summer precipitation as compared to precession and ice sheets.