Abstract: Analysis of plant photosynthesis and post-photosynthetic fractionation can improve our understanding of plant physiology and water management. By measuring δ¹³C in the atmosphere, and δ¹³C of soluble compounds in leaves and branch phloem of Platycladus orientalis, we examined discrimination pattern, including atmosphere-leaf discrimination during photosynthesis (ΔC_a-leaf) and leaf-twig discrimination during post-photosynthesis (ΔC_leaf-phlo), in response to changes of soil water content (SWC) and atmospheric CO₂ concentration (C_a). The results showed that ΔC_a-leaf reached a maximum of 13.06‰ at 95%-100% field water-holding capacity (FC) and C_a 400 μmol·mol^-1, and a minimum of 8.63‰ at 35%-45% FC and C_a 800 μmol·mol^-1. Both stomatal conductance and mesophyll cell conductance showed a significant linear positive correlation with ΔC_a-leaf, with a correlation coefficient of 0.43 and 0.44, respectively. ΔC_leaf-phlo was not affected by SWC and C_a. Our results provide mechanism of carbon isotopes fractionation and a theoretical basis for plant survival strategies in response to future climate change.