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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. Plain Language Summary: The East Asian summer monsoon (EASM) is a wind which carries moist air from the Indian and Pacific Oceans to East Asia, and it affects the economy and life of a large population. On astronomical time scale, its variation is controlled by insolation, CO2, and ice sheets. Here we use a Gaussian emulator based on climate simulations to assess how these factors affect the variation of the summer precipitation in East Asia. In the northern part of the EASM domain, the summer precipitation has a strong 23‐ky cycle. It is mainly controlled by precession. The effect of insolation is further modulated by ice sheets by the variations of the land‐sea pressure gradient. In the southern part, ice sheets play a vital role on the summer precipitation via its control on the location of the Intertropical Convergence Zone. Obliquity and CO2 have little effect on the precipitation. Key Points: In northern East Asia, the variation of summer precipitation is dominated by precessionIn southern East Asia, ice volume plays a more important role on the summer precipitation via its control on the location of Intertropical Convergence ZoneThe effects of obliquity and CO2 on summer precipitation are very small compared to precession and ice volume [ABSTRACT FROM AUTHOR]