Carbon isotope composition (δ13C) of bulk organic matter in soil and lake sediment sequences has been widely used to infer past climate changes. The specific climatic representation of the isotopic signature, however, lacks for examination. With excellent gradients of temperature and precipitation, as well as the gradients of vegetation zone, central East Asia provides an ideal background for developing the relationship between the δ13C and the climatic parameters. Analysis of surface soil organic matter from a transect that crosses the various climatic and vegetation zones reveals a significant linear correlation between δ13C and the mean precipitation from May to September yet a very poor relationship between δ13C and temperature. The close correlation between the δ13C and the precipitation is also corroborated by measurement of some C3 shrubs along the transect. Precipitation in the arid central East Asia is a key factor controlling the relative competition or the distribution of plant species with different photosynthetic pathways, thus the carbon isotope signature in the bulk organic matter. Its variation, therefore, can be recovered by measurement of δ13C in soils or lake sediment sequences with terrestrial organic matters.