Ultra high performance liquid chromatography (UHPLC) coupled with LTQ Orbitrap mass spectrometry was applied for profiling the phenolic constituents in a water extract of Lithocarpus polystachyus. The constituents were identified by combining their accurate m/z values of [M - H](-) ions and MSn spectra, as well as those data provided by literature. Sixty-eight phenolic compounds, including flavonoids, hydroxybenzoic acid, and hydroxycinnamic acid derivatives, were identified or tentatively characterized. Fragmentation behaviours of the flavonoids, especially those of the dihydrochalcone derivatives, were investigated thoroughly. More than half of the phenolic compounds identified had not been reported previously from this plant. Besides, 18 phloretin conjugates were tentatively identified as new compounds. This established UHPLC-PDA/ESI-MSn method is effective for identifying phenolic constituents and could be the basis for the comprehensive quality control of L. polystachyus.