We report glacial varves in the sediment of Lake Xinluhai, Tibetan Plateau. Independent data of 137Cs and 210Pb indicate that these are annually deposited varves. Varves appear as rhythmic units of light-colored silt layer capped by a dark clay layer under microscope. Varve thickness in Lake Xinluhai is sensitive to precipitation because sediment accumulation is strongly affected by monsoon rainfall in the area. A general decreasing trend can be observed in the varve thickness over the past 160 years. Spectral analyses of the varve record are dominated by cycles which are similar to ENSO periodicities. It implies that the decreasing trend of the South Asia monsoon may be linking with ENSO. Spatially, the decreasing trend can be observed across different proxy records in the south of the Tibetan Plateau. Although arguments still remain for the dynamic mechanisms and spatial rainfall difference, the South Asian summer monsoon could be weakened due to rising temperatures.