Two addition orders, i.e., the layer-by-layer (L) and mixed biopolymer (M) orders, were used to generate sodium caseinate - sugar beet pectin electrostatically stabilized o/w emulsions with 0.5% oil and varying sodium caseinate: sugar beet pectin ratios (3:1-1:3) at pH 4.5. Emulsion stability against environmental stresses (i.e., pH, salt addition, thermal treatment, storage and in vitro simulated gastrointestinal digestion) and its astaxanthin encapsulation against degradation during storage and in vitro digestion were evaluated. Results indicated that a total biopolymer concentration of 0.5% was optimal, with the preferred sodium caseinate-sugar beet pectin ratios for L and M emulsions being 1:1 and 1:3, respectively. L emulsions generally exhibited smaller droplet diameters than M emulsions across all ratios, except at 1:3. Lowering the pH to 1.5 substantially reduced the net negative charge of all emulsions, with only L emulsions precipitating at pH 3. M emulsions showed greater tolerance to salt addition, remaining stable up to 500 mM sodium and calcium concentrations, whereas L emulsions destabilized at levels exceeding 50 mM and 30 mM, respectively. All emulsions were stable when heated at 37 °C or 90 °C for 30 min. Astaxanthin degradation rates increased with prolonged storage, reaching 61.66% and 54.08% by day 7 for L and M emulsions, respectively. Encapsulation efficiency of astaxanthin in freshly prepared M emulsions (86.85%) was significantly higher compared to L emulsions (72.82%). M emulsions had 30% and 25% higher encapsulation efficiency of astaxanthin than L emulsions after in vitro digestion for 120 min and 240 min respectively. This study offers suggestions for interface design and process optimization to improve the performance of protein-polysaccharide emulsion systems, such as in beverages and dairy products, as well as their delivery effect of bioactives.