Plant polyphenols, known as “Category 7th nutrients” for human health, have received widespread attention in many fields, including medicine, food and nutrition. However, red-fleshed kiwifruit peel (RKP), which contains many polyphenols and is known as an excellent raw material for extracting plant polyphenols, is often discarded as a by-product. First, the polyphenols were extracted by pulsed ultrasound (PU) assisted with natural deep eutectic solvent (NADES) from RKP. Subsequently, protocatechuic acid (PCA), the critical polyphenolin RKP, was selected to investigate the inhibition effect on low-density lipoprotein (LDL) oxidation and to investigate the interaction mechanism with bovine serum albumin (BSA) by fluorescence spectroscopy and UV-Vis spectroscopy. The results were as follows: The extraction rate of polyphenols by NADES was significantly higher than that of conventional solvents. Among the 6 kinds of NADES solvents screened, the highest extraction rate of polyphenols from RKP (29.84 mg GAE/g DW) was obtained with choline chloride-ethylene glycol under the conditions of ultrasonic power of 400 W, material-to-liquid ratio of 1∶40 (g·mL-1), temperature of 70℃, extraction time of 20 min and water content of 20% (ω/ω). PCA had a strong scavenging ability on DPPH radicals, with the highest scavenging rate of 94.39% in the results of the spectroscopic experiments. PCA could significantly prolong the delay time of conjugated diene (CD) production and peak value during LDL oxidation, effectively inhibit the production of lipofuscin and total fluorescent products during lipid oxidation and reduce the oxidation of tryptophan (Trp) residues and oxidative modification of lysine (Lys) residues during LDL oxidation. These spectroscopic experiments showed a strong inhibition effect on LDL oxidation. The interaction between PCA and BSA was studied, and the following results were obtained using multispectral techniques. The interaction and the strong affinity between PCA and BSA occurred; there was only one binding site, and the hydrophobic force played a major role in the interaction process. The microenvironment around Tyrosine (Tyr) was almost unchanged after the interaction, while a reduction of polarity and enhanced hydrophobicity around Trp residues occurred. The interaction between these two molecules was further verified in the three-dimensional fluorescence spectra, and the protein structure of BSA was changed after this interaction occurred. The quenching mechanism of interaction between PCA and BSA was static quenching according to these fluorescence and UV-Vis spectra experiments. This study provides new insights for the development of RKP and PCA.