Background: Salvianolic acid B (SalB) represents the most abundant and bio‑active phenolic constituent among the water‑soluble compounds of Salvia miltiorrhiza. But the therapeutic potential of SalB has been significantly restricted by its poor absorption. Methods: In this study, chitosans (CS) and CS nanoparticles (NPs) with different molecular weights (MWs), which have influence on the absorption of SalB, was also investigated. Results: As a preliminary study, water‑soluble CS with various MWs (3, 30, 50, and 100 kDa) was chosen. We investigated the MW‑dependent Caco‑2 cell layer transport phenomena in vitro of CS and NPs at concentrations (4 μg/ml, w/v). SalB, in presence CS or NPs has no significant toxic effect on Caco‑2 cell. As the MW increases, the absorption enhancing effect of CS increases. However, as the MW decreases, the absorption enhancing effect of NPs increases. The AUC0–∞ of the SalB‑100 kDa CS was 4.25 times greater than that of free SalB. And the AUC0–∞ of the SalB‑3 kDa NPs was 16.03 times greater than that of free SalB. Conclusion: CS and NPs with different MWs as the absorption enhancers can promote the absorption of SalB. And the effect on NPs is better than CS.