Abstract: The identification and traceability of Apocynum venetum have long been hampered by two major problems. First, Apocynum venetum’s appearance and infrared spectrum is extremely similar to those of flax and other fiber plants, leading to insufficient discrimination by traditional methods. Second, the processing significantly alters the material composition of Apocynum venetum, causing interference in traceability. This study investigated Apocynum venetum products at different processing stages, with flax as the control sample, and uses an elemental analyzer-isotope ratio mass spectrometer (EA-IRMS) to accurately determine their carbon stable isotopes ratio (δ¹³C), hydrogen stable isotopes ratio (δD), and oxygen stable isotopes ratio (δ¹⁸O). Furthermore multiple statistical methods, including principal component analysis (PCA), cluster analysis, and partial least squares discriminant analysis (PLS-DA), were employed for data interpretation. The results show that flax from different origins presents unique isotopic fingerprints. These fingerprints not only effectively distinguish processing methods and blending situations but also accurately trace the raw material species and origin. This paper systematically compares the isotopic variation patterns of Apocynum venetum under different processing techniques, confirming that stable isotope technology can effectively distinguish Apocynum venetum and its products, identify processing methods, and trace the origin of raw materials. This work provides a new technical approach and key data support for the scientific traceability and authenticity identification of natural textiles.