The South Asian Summer Monsoon (SASM) plays a key role in controlling precipitation in the South Asia. Orbital and millennial SASM precipitation changes have been, respectively, linked to boreal high-latitude summer insolation and North Atlantic climate. However, causes for the heterogenous spatial SASM precipitation patterns remain debated. Here, we report SASM precipitation histories based on multiproxy evidence from the northwest South China Sea during the past 40 ka. Using fine-grained magnetic minerals as a proxy for precipitation, we find that SASM precipitation in Southwest China varied following the local insolation with muted millennial-scale fluctuations, contrasting to evolutions in other Asian monsoon regions. Transient model simulations reveal that local orbital insolation generated a tripole precipitation pattern over the East Asia by influencing atmospheric circulation during the early last deglaciation, with high orbital insolation corresponding to increased precipitation in Southwest China. Our study highlights influences of local insolation on East Asian precipitation patterns on orbital and millennial timescales.