Galectin-3 (Gal-3) is a new target for tumor therapy, which exerts anti-tumor effects through the specific binding with the modified pectin fragments. In this study, the pectin from citrus peel was directionally degraded by ultraviolet light catalytic hydrogen peroxide oxidation in synergy with acid or alkaline, resulting in eight degraded pectin products. The structural parameters of the degraded products, including molecular weight, degree of methylation and degree of acetylation were determined. The activity-guided separation was accomplished based on the specific binding between the active pectin fragments and Gal-3. The active pectin fragment UHP 4-5-0.3, having the minimum inhibition concentration (MIC) of 0.016 mg/mL, was screened out through the hemagglutination assay and column chromatography separation. The CCK-8 anti-proliferation activity assay revealed that the proliferation inhibition rates of UHP 4-5-0.3 against the three types of Gal-3 highly-expressed tumor cells, HepG2, MCF-7 and HeLa, reached 72.8 %, 90.0 %, and 85.7 %, respectively. The structure of the active pectin fragment was determined to be composed of a highly branched rhamnogalacturonan I and a linear homogalacturonan by FT-IR, 1H NMR and HSQC analyses. The findings of this study provide a new idea for efficiently obtaining a novel food functional ingredient from pectin.