Polysaccharides from functional foods have been proved to have diverse bioactivities, but little is known about what exactly happens to these polysaccharides after oral administration and even less about the underlying mechanism of action. Taking the marker polysaccharide (DOP) of Dendrobium officinale as an example, this study aims to demonstrate the dynamic distribution and degradation of orally dosed DOP in mice and in vitro using near-infrared fluorescence imaging and a kind of chromatographic analysis. The results indicate that (1) neither DOP nor fluorescence-labeled DOP (FDOP) was absorbed, (2) both DOP and FDOP were undigested and were quickly degraded to short-chain fatty acids in the large intestine, (3) DOP modulated gut microbiota, which could be associated with DOP's suppression of 4T1 tumor growth in mice. All of these findings suggest that some (maybe not all) bioactive polysaccharides share a common destiny: indigestible and nonabsorbing, ends in modulating bioactivities-associated gut microbiota.