Abstract: To enhance the bioaccessibility of astaxanthin (ASTA), this study constructed astaxanthin nanoemulsions (ASTA-NEs) using high-pressure microfluidization technology, with methyl linolenate (ML, esterified modification), flaxseed oil (FO, rich in unsaturated fatty acids), and coconut oil (CO, rich in saturated fatty acids) as the oil phases. The prepared systems included astaxanthin-loaded methyl linolenate nanoemulsion (MLA), astaxanthin-loaded flaxseed oil nanoemulsion (FOA), and astaxanthin-loaded coconut oil nanoemulsion (COA), with astaxanthin-free methyl linolenate emulsion (ML) serving as the control. By characterizing the emulsion structure, stability, in vitro digestive behavior, and ASTA bioaccessibility, the regulatory mechanism of oil phase composition on the performance of the delivery system was revealed. Results showed that all emulsions exhibited a single narrow-peak distribution, with a polydispersity index (PDI) below 0.2 and negative Zeta potential, indicating stable and homogeneous systems. Among them, MLA had the optimal dispersion uniformity. The stability of emulsions with different oil phases was affected by pH, Na⁺ concentration, and temperature. Within the pH range of 5~9, MLA showed a smaller PDI value (P<0.05). For ionic stability, in the range of 0~300 mmol/L NaCl, the particle size of ML increased significantly with the increase of Na⁺ concentration (P<0.05), while the particle sizes of MLA, FOA, and COA remained stable. In addition, the thermal stability of different samples was evaluated within the temperature range of 30~90 ℃: at 30~60 ℃, MLA showed no significant change in particle size (P>0.05) and exhibited the best thermal stability. At 90 ℃, the particle sizes of all samples increased, but MLA still maintained a relatively small particle size (<195 nm). During in vitro digestion, the emulsion structure remained stable in the oral and gastric phases. In the intestinal phase, the particle size of COA increased significantly (P<0.05) due to the rapid hydrolysis of high saturated fatty acids. After digestion, MLA had the highest ASTA retention rate, with a bioaccessibility of 26.8%, which was significantly higher than that of FOA (24.8%) and COA (22.4%) (P<0.05). This study clarifies the regulatory mechanism of oil phase properties on the performance of astaxanthin nanoemulsions, providing a theoretical basis for the rational design and industrial application of functional lipid delivery systems.