This study sampled 20-year-old Robinia pseudoacacia plantations at 10 typical sites on the Loess Plateau. We determined the C, N and P concentrations in Robinia pseudoacacia leaves and various physical and chemical indexes of 0-10 cm and 10-20 cm soil layers, and further explored the variations in the ecological stoichiometric parameters of the leaf-soil system along a water gradient on the Loess Plateau. The results showed that: ① leaf C and N concentrations increased with increasing rainfall, but there was no clear trend for leaf P concentration along a rainfall gradient. ② Soil organic carbon (SOC), total nitrogen (TN), total phosphorus (TP), and ammonium nitrogen concentrations and soil C:P and N:P ratios increased with increasing rainfall in 0-10 cm soil layer, but soil C:N has no obvious change. ③ There were no significant correlations between plant and soil C, N, P concentrations and C:N, C:P, N:P ratios, but leaf N and P concentrations were significantly positively correlated with soil available phosphorus and ammonium concentrations. The result of leaf N:P>16 indicated that the plant was under P limitation on the Loess Plateau. Additionally, the soil P limitation increased with increasing rainfall.