This study investigated the humidity/aridity gradient effects and ecological stoichiometry in Xishuangbanna tropical rainforest. The geographical pattern, characteristics of ecological stoichiometry and its response to altitude and humidity/aridity effects were studied in the soil and plants of Xishuangbanna tropical rainforest using field and laboratory analyses. The results showed that the ecological stoichiometry of soil and leaves in Xishuangbanna rainforest was influenced by altitude and humidity gradients, to varying degrees. The hydrothermal gradient of seasonal and montane rainforests was affected by altitude. The soil moisture content had a significant effect on soil organic carbon (SOC) (P < 0.01) in dry and wet seasons, which was significantly controlled by the altitude gradient. The effect of soil moisture content on soil total nitrogen (STN) and soil total phosphorus (STP) was more significant during the rainy season. The total phosphorus (TP) content in leaves increased with increasing water content, and the variation trend of leaf TN content in the dry season was consistent with the TP content. However, water content reduced the total nitrogen (TN) content when it increased to a certain extent during the rainy season. The significant correlation among soil moisture in the dry season, altitude, and the C/P of soil, similar to the correlation between C/N and water content of leaves in the dry season, indicated that the level of soil water content determines the mineralization of soil phosphorus and the ability of plants to absorb P under the water shortage. Leaf C/N has obvious implications for the feedback moisture content of plants. This further illustrates that the hydrothermal gradient is an important driver for the stoichiometry characteristics of soil-leaf ecosystems. Additionally, high temperature and rain may weaken the assimilation capacity of leaf C content in a seasonal rainforest, and leaf N content may decrease in terms of regional response to global change. Furthermore, the elemental cycles of soil-leaf systems would be shortened, and the dry season may weaken the limiting effect of P in plant ecosystems of montane rain forests because of the coupling effects of P cycles to other elements.