Cydonia oblonga miller (quince) plant serves as a potential folk medicine for treating hypertension and cardiovascular diseases in China. However, to the best of our knowledge, no study has been conducted on the polyphenolic profile and anti-adipogenic effect of quince fruit grown in China. In the current study, we aimed to investigate the quince fruit extract’s major phenolic compounds, evaluate their antioxidant activity, and examine their effect on adipogenesis in 3T3-L1 cells. A rapid and sensitive analytical method was established for the simultaneous determination of major polyphenolic compounds by using ultra-pressure liquid chromatography coupled with a triple quadrupole mass spectrometer (UPLC-MS/MS). Among the 10 compounds, the cryptochlorgenic acid was noticed as the most abundant compound of both purified (242.44 ± 0.73 µg/mg dw) and unpurified extract (3.37 ± 0.01 µg/mg dw) followed by quercetin 3-rutinoside and chlorogenic acid. Alternatively, both extracts possessed a high quantity of phenolic acids (purified extract = 483.10 ± 5.16 µg/mg dw and unpurified extract = 7.89 ± 0.02 µg/mg dw). The purified extract exhibited a strong antioxidant capacity (DPPH: EC50 = 3.316 µg/mL, ABTS: EC50 = 36.38 µg/mL) as compared to the unpurified extract. Additionally, our results also showed that the extract at 100 µg/mL significantly suppressed the preadipocyte differentiation and decreased the lipid droplets up to 69% in mature adipocytes. The present study highlights an accurate and fast detection method for quince fruit extract polyphenolic compounds with its antioxidant and antiadipogenic effects. The study also provides the necessary information for the rational development and utilization of quince fruit extract as a source of phytochemicals.