Flavonoids are secondary metabolites that are widely distributed in plants with numerous biological functions. This study focused on Liquidambar formosana, a tree species rich in flavonoids that are used for ornamental and medicinal purposes. To better understand the chemical processes leading to seasonal variation in leaf color in L. formosana ‘Nanlinhong,’ we selected four colors of leaves in three seasons from the same individual trees, bright red in early spring, red in middle spring, green in summer, and dark red in autumn. The three-year-old trees of this variety were grown in the greenhouse under natural light with day and night temperatures from 30 to 23 ℃, and fully expanded leaves from the top of each individual were employed in flavonoid analyses. The results showed that the total flavonoid content was highest in spring, while anthocyanidin content was highest in autumn leaves. A total of 148 flavonoid metabolites were found using the Ultra-Performance Liquid Chromatography–Electrospray Ionization–Tandem Mass Spectrometry (UPLC-ESI–MS/MS) method; the compounds comprised 58 flavones, 33 flavonols, 18 flavanones, 16 polyphenols, 15 anthocyanins, six isoflavones, and two proanthocyanins. Through clustering analysis, a total of 122 types of metabolites were found in spring with higher expression levels, and 104 metabolites were found in autumn, indicating that flavonoids were more predominant in spring. To date, only 23 flavonoid compounds had been identified in this tree species, but the present study revealed 100 new compounds. In addition, our findings provide strong support for a differentiated use of L. formosana ‘‘s leaves from different seasons in targeting a wide range of medical conditions.