An ultra-high performance liquid chromatography-triple quadrupole mass spectrometry (UPLC-QQQ-MS) method was established to simultaneously determine the content of three monosaccharides in Dendrobium denneanum and D. officinale, and construct a system with monosaccharide components as an indicator to evaluate the quality of D. denneanum and D. officinale from different sources. The results show that the three monosaccharide components have a good linear relationship within a certain concentration range, the correlation coefficients are all greater than 0.999, the stability, precision, and repeatability RSD are all less than 4.0%, and the sample recovery rate is 99.62%~102.31%, with RSD between 1.49% and 2.67%. Both D. denneanum and D. officinale polysaccharides contain three monosaccharide components: glucose, mannose, and galactose, but there are certain differences in content. The UPLC-QQQ-MS method established in this study is fast, accurate, efficient, and reproducible. It can provide a reference for the quality evaluation of D. denneanum and D. officinale, and provide technical support for the further development and utilization of its resources.