Cow’s milk protein allergy is a common type of food allergy. As one of the major allergens in cow’s milk, β-lactoglobulin (βLG) exhibits high structural stability and resistance to digestion. Therefore, improving its digestibility and reducing its immunoreactivity are of considerable importance. In this study, covalent conjugates of βLG with gallic acid (GA), caffeic acid (CFA), cinnamic acid (CIA), and ferulic acid (FA) were prepared using a free-radical-induced method. Their structural characteristics, thermal stability, antibody-binding capacities, and simulated gastrointestinal digestibility were subsequently evaluated. The molecular weights of the conjugates increased by approximately 110–130 Da compared with native βLG, accompanied by a significant decrease in free amino group content. The maximum fluorescence emission wavelength shifted from 334.4 nm for native βLG to 335.4–336.2 nm for the conjugates, indicating partial unfolding of the protein structure. The thermal denaturation temperature of native βLG was 82.4 °C and shifted toward lower temperatures after covalent modification. The IgE-binding capacities of βLG–GA, βLG–CFA, βLG–CIA, and βLG–FA decreased by 41.7%, 32.3%, 26.0%, and 21.9%, respectively, whereas their IgG-binding capacities decreased by 11.0%, 15.4%, 15.4%, and 9.6%, respectively. After 120 min of simulated gastric digestion, the protein bands of the conjugates showed greater attenuation than those of native βLG. After 60 min of intestinal digestion, the intact βLG bands were nearly undetectable. These results demonstrate that covalent conjugation with phenolic acids loosens the βLG structure, improves its in vitro digestibility, and reduces its antibody-binding capacity, with GA showing the strongest inhibitory effect on IgE binding. This study provides a theoretical basis for the development of low-allergenicity milk proteins and related food-processing strategies using natural phenolic acids.