Citrus grandis cv. ‘Tomentosa’ is an important traditional Chinese medicine agent; naringin and other flavonoids are the main functional and quality control compounds. We established methods for extraction, high-performance liquid chromatography (HPLC) quantitative and fingerprint analyses of flavonoid content of C. grandis cv. ‘Tomentosa’ and analyzed dynamic changes in content during fruit growth and development. The powder from fruit was extracted with 60% ethanol for 2 h by ultrasound-assisted maceration extraction. The extraction of naringin reached ≥98%. Separation and quantitative determination of flavoloids and fingerprint analysis of extracts by HPLC involved Thermo C18 columns and a gradient eluent of 35% to 65% acidic methanol. Content of the 4 main flavonoids was analyzed every 5 days from days 15 to 60. Total flavonoid content increased significantly with fruit age from 0.55 to 7.99 g·fruit^–1. However, on a fruit dry-weight basis, content of flavonoid decreased significantly with fruit age and that of naringin from 52.5% to 16.1%, rhoifolin from 0.74% to 0.11%, naringenin from 0.57% to 0.06%, and apigenin from 0.23% to 0.03%. Fruit age had a great impact on fruit flavonoid content, yield and quality of medicinal material from fruit. We provide important information for industrial production and quality control of C. grandis cv. ‘Tomentosa’.