Inulin-type fructooligosaccharides (FOS) purified from Morinda Officinalis, with degrees of polymerization (DP) from 3 to 9, have been approved in China as an oral prescribed drug for mild and moderate depression episode, while the stability and oral absorption of this FOS mixtures are largely unknown. As the main active component and quality control marker for above FOS, DP5 was selected as the representative FOS in this study. Desalting method by ion exchange resin was developed to treat bio-sample, followed by separation and quantification by high performance liquid chromatography-charged aerosol detector. Results showed that the DP5 was stepwisely hydrolyzed in simulated gastric fluid and gut microbiota, while maintained stable in intestinal fluid. DP5 has poor permeability across Caco-2 monolayer with P_app of 5.22 × 10⁻⁷ cm/s, and very poor oral absorption with bioavailability of (0.50 ± 0.12)% in rat. In conclusion, FOS in Morinda Officinalis demonstrated poor chemical stability in simulated gastric fluid and human gut microbiota, and low oral absorption in rats.