Dendrobium officinale is a traditional Chinese medicinal herb that produces promising bioactive polysaccharides. However, the biosynthetic pathway of polysaccharides in this herb remains to be elucidated. The uridine diphosphate glucose pyrophosphorylase (UGPase) is a key enzyme for the production of uridine diphosphate glucose, which is a major glycosyl donor for synthesis of polysaccharides. This study identified a novel UGPase gene from D. officinale termed as DoUGP. Bioinformatics and subcellular-localization of the DoUGP protein indicate that it belongs to the UGPase-A type and was localized in cytoplasm. The DoUGP was revealed to be constitutively expressed in all organs, and the highest mRNA content was detected in stems, the organs with the highest polysaccharide content. Furthermore, sucrose feeding experiments in D. officinale demonstrate that sucrose addition could increase DoUGP transcription significantly and enhance polysaccharide accumulation accordingly. Together, we conclude that DoUGP probably plays an important role in polysaccharide biosynthesis of D. officinale and is a potential target for quality breeding of this orchid.