Global climate change has a profound impact on forest ecosystems. Stable isotopes in tree rings are an important tool in studies of tree response to environmental factors. In this study, four tree-ring stable carbon isotope de-trended series (δ¹³C_corr) from southern and northern slopes of the Tianshan Mountains were established using tree ring samples collected from the Aksu River Basin on the southern slope of the Tianshan Mountains, the Ili River Basin and the Issyk-Kul Basin in Kyrgyzstan on the northern slopes of the Tianshan Mountains according to the stable carbon isotope methods. Analysis shows that the tree-ring stable carbon isotope values on the northern slope are lower but more stable than those on the southern slope of Tianshan Mountain. The tree-ring stable carbon isotope on the southern slope has shown an obvious negative trend, especially after the 1970s, while the northern slope is relatively stable over the past 160 years. Further, the difference in the response of tree-ring stable carbon isotopes to climate on the northern and southern slopes of Tianshan Mountains is discussed. Correlation function and collinearity analysis are used to reveal the relationship between the tree-ring δ¹³C_corr and various climatic factors. The results show that the tree-ring δ¹³C_corr on the southern slope of Tianshan Mountains is significantly negatively correlated with precipitation and relative humidity during the growing season, but has no obvious correlation with temperature. It indicates that the main climatic factor affecting the stable isotope fractionation of tree rings on the southern slope of Tianshan Mountains is the moisture condition in the growing season, especially precipitation. The tree-ring δ¹³C_corr on the northern slope of Tianshan Mountain is significantly positively correlated with the mean temperature, mean maximum temperature and vapor pressure deficit in the growing season, especially in summer, and negatively correlated with precipitation and relative humidity. These results indicating that the influencing climate factors of tree-ring δ¹³C_corr may be more complicated on the northern slope of Tianshan Mountain. Further analysis shows that summer temperature, precipitation and relative humidity jointly regulate the tree-ring stable isotope fractionation on the northern slope of Tianshan Mountains. Overall, the climate factors which influence tree-ring stable carbon isotope fractionation on the southern slope is more than that on the northern slope of Tianshan Mountain. The main limiting factors affecting the tree-ring stable carbon isotope fractionation on the southern slope are precipitation during the growing season and the synergistic effect of precipitation and relative humidity. While the process of tree-ring stable isotope fractionation on the northern slope of Tianshan Mountain is synergistic effect by the summer mean maximum temperature, precipitation and relative humidity.