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Long-term forest productivity and carbon sequestration capacity are affected by climate change, and have become a global concern. In this study, we provide a simple and nondestructive method to determine tree CO2 assimilation across multiple time scales. The new method combines sap flow and stable carbon isotope discrimination (Delta C-13) measurements to estimate carbon assimilation. We analyzed variability and conducted paired-sample t-tests to compare CO2 assimilation estimated from gas exchange measurements and the new approach to verify its accuracy and applicability. Gas exchange and isotopic measurements both showed that the CO2 assimilation rate in the morning was higher than in the afternoon, and peak values occurred around 10-11 a.m., which may have been due to nocturnal water storage and higher stomatal conductance in the morning. Diurnal, monthly, and annual CO2 assimilation of P. orientalis variations were related to water supply conditions. Compared with previous research, we experimented using tree-ring stable carbon isotope discrimination (Delta C-13) and sap flow measurement to estimate annual CO2 assimilation and the results were consistent with other traditional methods. Platycladus orientalis is effective and responsive to water supply, which explained why it adapts well in semi-arid areas. The new method for estimating CO2 assimilation was accurate and applicable in semi-arid areas around Beijing.